TYP: Overload concat
- [ ] Closes part of MarcoGorelli/pyladieslondon-sprints#29
- [ ] Adds typing.overload to: `concat` from https://github.com/pandas-dev/pandas/blob/master/pandas/core/reshape/concat.py

## `mypy` Setup and Sutput
```python
# t.py
import pandas as pd
from pandas._typing import Axis
from pandas.core.generic import NDFrame

axis: Axis

ndf1: NDFrame
ndf2: NDFrame

s1 = pd.Series
s2 = pd.Series

df1 = pd.DataFrame
df2 = pd.DataFrame

reveal_type(pd.concat([s1, s2]))
reveal_type(pd.concat([s1, s2], axis=0))
reveal_type(pd.concat([s1, s2], axis=1))
reveal_type(pd.concat([s1, s2], axis="index"))
reveal_type(pd.concat([s1, s2], axis="columns"))

reveal_type(pd.concat([df1, df2]))
reveal_type(pd.concat([df1, df2], axis=0))
reveal_type(pd.concat([df1, df2], axis=1))
reveal_type(pd.concat([df1, df2], axis="index"))
reveal_type(pd.concat([df1, df2], axis="columns"))

reveal_type(pd.concat([ndf1, ndf2]))
reveal_type(pd.concat([ndf1, ndf2], axis=0))
reveal_type(pd.concat([ndf1, ndf2], axis=1))
reveal_type(pd.concat([ndf1, ndf2], axis="index"))
reveal_type(pd.concat([ndf1, ndf2], axis="columns"))
```
which results in the following mypy output:
```bash
$ mypy t.py
  t.py:16: note: Revealed type is 'pandas.core.series.Series'
  t.py:17: note: Revealed type is 'pandas.core.series.Series'
  t.py:18: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:19: note: Revealed type is 'pandas.core.series.Series'
  t.py:20: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:22: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:23: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:24: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:25: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:26: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:28: note: Revealed type is 'Union[pandas.core.frame.DataFrame, pandas.core.series.Series]'
  t.py:29: note: Revealed type is 'Union[pandas.core.frame.DataFrame, pandas.core.series.Series]'
  t.py:30: note: Revealed type is 'pandas.core.frame.DataFrame'
  t.py:31: note: Revealed type is 'Union[pandas.core.frame.DataFrame, pandas.core.series.Series]'
  t.py:32: note: Revealed type is 'pandas.core.frame.DataFrame'
```