Local test failure pandas.tests.frame.test_analytics
Running 

```
pytest -q --cache-clear pandas/tests/frame/test_analytics.py
```

Yield the following error:

<details>

```
___________________________ TestDataFrameAnalytics.test_mean_mixed_string_decimal ____________________________
TypeError: unsupported operand type(s) for +: 'int' and 'str'

The above exception was the direct cause of the following exception:

self = Decimal('3351.00'), other = Decimal('572.00'), context = None

    def __add__(self, other, context=None):
        """Returns self + other.
    
        -INF + INF (or the reverse) cause InvalidOperation errors.
        """
>       other = _convert_other(other)

/usr/lib/python3.8/_pydecimal.py:1158: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

other = Decimal('572.00'), raiseit = False, allow_float = False

    def _convert_other(other, raiseit=False, allow_float=False):
        """Convert other to Decimal.
    
        Verifies that it's ok to use in an implicit construction.
        If allow_float is true, allow conversion from float;  this
        is used in the comparison methods (__eq__ and friends).
    
        """
>       if isinstance(other, Decimal):
E       SystemError: <built-in function isinstance> returned a result with an error set

/usr/lib/python3.8/_pydecimal.py:6017: SystemError

The above exception was the direct cause of the following exception:

self = Decimal('3351.00'), other = Decimal('609.00'), context = None

    def __add__(self, other, context=None):
        """Returns self + other.
    
        -INF + INF (or the reverse) cause InvalidOperation errors.
        """
>       other = _convert_other(other)

/usr/lib/python3.8/_pydecimal.py:1158: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

other = Decimal('609.00'), raiseit = False, allow_float = False

    def _convert_other(other, raiseit=False, allow_float=False):
        """Convert other to Decimal.
    
        Verifies that it's ok to use in an implicit construction.
        If allow_float is true, allow conversion from float;  this
        is used in the comparison methods (__eq__ and friends).
    
        """
>       if isinstance(other, Decimal):
E       SystemError: <built-in function isinstance> returned a result with an error set

/usr/lib/python3.8/_pydecimal.py:6017: SystemError

The above exception was the direct cause of the following exception:

self = Decimal('3351.00'), other = Decimal('820.00'), context = None

    def __add__(self, other, context=None):
        """Returns self + other.
    
        -INF + INF (or the reverse) cause InvalidOperation errors.
        """
>       other = _convert_other(other)

/usr/lib/python3.8/_pydecimal.py:1158: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

other = Decimal('820.00'), raiseit = False, allow_float = False

    def _convert_other(other, raiseit=False, allow_float=False):
        """Convert other to Decimal.
    
        Verifies that it's ok to use in an implicit construction.
        If allow_float is true, allow conversion from float;  this
        is used in the comparison methods (__eq__ and friends).
    
        """
>       if isinstance(other, Decimal):
E       SystemError: <built-in function isinstance> returned a result with an error set

/usr/lib/python3.8/_pydecimal.py:6017: SystemError

The above exception was the direct cause of the following exception:

self = <pandas.tests.frame.test_analytics.TestDataFrameAnalytics object at 0x612b1fcb3700>

    def test_mean_mixed_string_decimal(self):
        # GH 11670
        # possible bug when calculating mean of DataFrame?
    
        d = [
            {"A": 2, "B": None, "C": Decimal("628.00")},
            {"A": 1, "B": None, "C": Decimal("383.00")},
            {"A": 3, "B": None, "C": Decimal("651.00")},
            {"A": 2, "B": None, "C": Decimal("575.00")},
            {"A": 4, "B": None, "C": Decimal("1114.00")},
            {"A": 1, "B": "TEST", "C": Decimal("241.00")},
            {"A": 2, "B": None, "C": Decimal("572.00")},
            {"A": 4, "B": None, "C": Decimal("609.00")},
            {"A": 3, "B": None, "C": Decimal("820.00")},
            {"A": 5, "B": None, "C": Decimal("1223.00")},
        ]
    
        df = pd.DataFrame(d)
    
>       result = df.mean()

pandas/tests/frame/test_analytics.py:499: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pandas/core/generic.py:11039: in stat_func
    return self._reduce(
pandas/core/frame.py:7876: in _reduce
    result = f(values)
pandas/core/frame.py:7828: in f
    return op(x, axis=axis, skipna=skipna, **kwds)
pandas/core/nanops.py:127: in f
    result = alt(values, axis=axis, skipna=skipna, **kwds)
pandas/core/nanops.py:560: in nanmean
    the_sum = _ensure_numeric(values.sum(axis, dtype=dtype_sum))
../../Venvs/venv-pandas/lib/python3.8/site-packages/numpy/core/_methods.py:38: in _sum
    return umr_sum(a, axis, dtype, out, keepdims, initial, where)
/usr/lib/python3.8/_pydecimal.py:1158: in __add__
    other = _convert_other(other)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

other = Decimal('1223.00'), raiseit = False, allow_float = False

    def _convert_other(other, raiseit=False, allow_float=False):
        """Convert other to Decimal.
    
        Verifies that it's ok to use in an implicit construction.
        If allow_float is true, allow conversion from float;  this
        is used in the comparison methods (__eq__ and friends).
    
        """
>       if isinstance(other, Decimal):
E       SystemError: <built-in function isinstance> returned a result with an error set

/usr/lib/python3.8/_pydecimal.py:6017: SystemError
1 failed, 90 passed, 1 xfailed in 1.89s
```

</details>

---

Output of ```pd.show_versions()```:

<details>

INSTALLED VERSIONS
------------------
commit           : f6b3e82b5514da97b48918c99bd28a07b754291b
python           : 3.8.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.23.a-1-hardened
Version          : #1 SMP PREEMPT Sun, 01 Mar 2020 18:23:23 +0000
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0.dev0+696.gf6b3e82b5
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : 0.29.15
pytest           : 5.3.5
hypothesis       : 5.6.0
sphinx           : 2.4.4
blosc            : 1.8.3
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.2
fastparquet      : 0.3.3
gcsfs            : None
matplotlib       : 3.2.0
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : 0.8.6
xarray           : 0.15.0
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.48.0


</details>