Multiline pd.eval() seems to be broken
#### Code Sample:

```python
In [93]: df
Out[93]:
   A  B
0  1  2
1  2  3
2  3  4
3  4  5

In [102]: pd.eval("""A = df.A - df.B
     ...: B = df.A + df.B
     ...: """,target=pd.DataFrame())

```
#### Problem description

```python
....
/usr/local/lib64/python3.6/site-packages/pandas/core/computation/scope.py in resolve(self, key, is_local)
    201                 from pandas.core.computation.ops import UndefinedVariableError
    202
--> 203                 raise UndefinedVariableError(key, is_local)
    204
    205     def swapkey(self, old_key: str, new_key: str, new_value=None):

UndefinedVariableError: name 'df' is not defined

```

#### Expected Output
```python
Out[106]:
   A  B
0 -1  1
1 -1  2
2 -1  3
3 -1  4
```

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

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 3.10.0-1062.9.1.el7.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 9.0.3
setuptools       : 45.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None



</details>
