Test for tests\indexes\datetimes\test_tools.py fails 
I'm getting an error when running the tests suite. I've locatet the relevant PR to be #26585.

It is failing on my machine, but not on the CI, so I assume some configuration differences. I'm on a Windows machine, which could be the relevant difference. See my cinfuíguration at the bottom of this issue.

```
================================================================= FAILURES ==================================================================
______________________ TestTimeConversionFormats.test_int_to_datetime_format_YYYYMMDD_typeerror[2012010101-2012010101] ______________________

self = <pandas.tests.indexes.datetimes.test_tools.TestTimeConversionFormats object at 0x000002950CC92E80>, int_date = 2012010101
expected = 2012010101

    @pytest.mark.parametrize('int_date, expected', [
        # valid date, length == 8
        [20121030, datetime(2012, 10, 30)],
        # short valid date, length == 6
        [199934, datetime(1999, 3, 4)],
        # long integer date partially parsed to datetime(2012,1,1), length > 8
        [2012010101, 2012010101],
        # invalid date partially parsed to datetime(2012,9,9), length == 8
        [20129930, 20129930],
        # short integer date partially parsed to datetime(2012,9,9), length < 8
        [2012993, 2012993],
        # short invalid date, length == 4
        [2121, 2121]])
    def test_int_to_datetime_format_YYYYMMDD_typeerror(self, int_date,
                                                       expected):
        # GH 26583
>       result = to_datetime(int_date, format='%Y%m%d', errors='ignore')

pandas\tests\indexes\datetimes\test_tools.py:152:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas\util\_decorators.py:188: in wrapper
    return func(*args, **kwargs)
pandas\core\tools\datetimes.py:626: in to_datetime
    result = convert_listlike(np.array([arg]), box, format)[0]
pandas\core\tools\datetimes.py:270: in _convert_listlike_datetimes
    arg, format, exact=exact, errors=errors)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

>   val[found.end():])
E   TypeError: 'int' object is unsliceable

pandas\_libs\tslibs\strptime.pyx:149: TypeError
________________________ TestTimeConversionFormats.test_int_to_datetime_format_YYYYMMDD_typeerror[20129930-20129930] ________________________

self = <pandas.tests.indexes.datetimes.test_tools.TestTimeConversionFormats object at 0x000002950D025C18>, int_date = 20129930
expected = 20129930

    @pytest.mark.parametrize('int_date, expected', [
        # valid date, length == 8
        [20121030, datetime(2012, 10, 30)],
        # short valid date, length == 6
        [199934, datetime(1999, 3, 4)],
        # long integer date partially parsed to datetime(2012,1,1), length > 8
        [2012010101, 2012010101],
        # invalid date partially parsed to datetime(2012,9,9), length == 8
        [20129930, 20129930],
        # short integer date partially parsed to datetime(2012,9,9), length < 8
        [2012993, 2012993],
        # short invalid date, length == 4
        [2121, 2121]])
    def test_int_to_datetime_format_YYYYMMDD_typeerror(self, int_date,
                                                       expected):
        # GH 26583
>       result = to_datetime(int_date, format='%Y%m%d', errors='ignore')

pandas\tests\indexes\datetimes\test_tools.py:152:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas\util\_decorators.py:188: in wrapper
    return func(*args, **kwargs)
pandas\core\tools\datetimes.py:626: in to_datetime
    result = convert_listlike(np.array([arg]), box, format)[0]
pandas\core\tools\datetimes.py:270: in _convert_listlike_datetimes
    arg, format, exact=exact, errors=errors)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

>   val[found.end():])
E   TypeError: 'int' object is unsliceable

pandas\_libs\tslibs\strptime.pyx:149: TypeError
_________________________ TestTimeConversionFormats.test_int_to_datetime_format_YYYYMMDD_typeerror[2012993-2012993] _________________________

self = <pandas.tests.indexes.datetimes.test_tools.TestTimeConversionFormats object at 0x000002950CEC7908>, int_date = 2012993
expected = 2012993

    @pytest.mark.parametrize('int_date, expected', [
        # valid date, length == 8
        [20121030, datetime(2012, 10, 30)],
        # short valid date, length == 6
        [199934, datetime(1999, 3, 4)],
        # long integer date partially parsed to datetime(2012,1,1), length > 8
        [2012010101, 2012010101],
        # invalid date partially parsed to datetime(2012,9,9), length == 8
        [20129930, 20129930],
        # short integer date partially parsed to datetime(2012,9,9), length < 8
        [2012993, 2012993],
        # short invalid date, length == 4
        [2121, 2121]])
    def test_int_to_datetime_format_YYYYMMDD_typeerror(self, int_date,
                                                       expected):
        # GH 26583
>       result = to_datetime(int_date, format='%Y%m%d', errors='ignore')

pandas\tests\indexes\datetimes\test_tools.py:152:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas\util\_decorators.py:188: in wrapper
    return func(*args, **kwargs)
pandas\core\tools\datetimes.py:626: in to_datetime
    result = convert_listlike(np.array([arg]), box, format)[0]
pandas\core\tools\datetimes.py:270: in _convert_listlike_datetimes
    arg, format, exact=exact, errors=errors)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

>   val[found.end():])
E   TypeError: 'int' object is unsliceable

pandas\_libs\tslibs\strptime.pyx:149: TypeError
================================================= 3 failed, 394 deselected in 2.40 seconds ==================================================
```

<details>

INSTALLED VERSIONS
------------------
commit: 649ad5c91aabb95d7f200eec84b82cae1459fc65
python: 3.7.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.25.0.dev0+616.g7f0a7435e
pytest: 4.3.0
pip: 18.1
setuptools: 40.6.2
Cython: 0.29
numpy: 1.16.4
scipy: 1.1.0
pyarrow: 0.11.1
xarray: None
IPython: 7.5.0
sphinx: 1.8.2
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: None
tables: 3.5.1
numexpr: 2.6.8
feather: None
matplotlib: 3.0.1
openpyxl: 2.5.12
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: None
lxml.etree: 4.3.3
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
