[BUG] Support Hashable type column

[Zeroto521]

Brief Description

The type of DataFrame's columns is support Hashable.
But Janitor only handles str type columns.

Related Source Codes List

• Fix bugs
  • janitor.utils.check_column
  • janitor.fill.fill_empty
• Update type annotations
  • janitor.utils.check_column
  • janitor.fill.fill_empty
  • janitor.functions.encode_categorical
  • and more

Minimally Reproducible Code

>>> import pandas as pd
>>> import janitor  # noqa

>>> df = pd.DataFrame({0:[None, None]})
>>> df.columns
Int64Index([0], dtype='int64')

>>> df.fill_empty(0, 0)
Traceback (most recent call last):
  File "C:\Software\miniforge3\envs\work\lib\site-packages\multipledispatch\dispatcher.py", line 269, in __call__
    func = self._cache[types]
KeyError: (<class 'pandas.core.frame.DataFrame'>, <class 'int'>)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Software\miniforge3\envs\work\lib\site-packages\pandas_flavor\register.py", line 29, in __call__
    return method(self._obj, *args, **kwargs)
  File "C:\Software\miniforge3\envs\work\lib\site-packages\janitor\utils.py", line 231, in wrapper
    return func(*args, **kwargs)
  File "C:\Software\miniforge3\envs\work\lib\site-packages\janitor\functions\fill.py", line 199, in fill_empty
    return _fill_empty(df, column_names, value=value)
  File "C:\Software\miniforge3\envs\work\lib\site-packages\multipledispatch\dispatcher.py", line 273, in __call__
    raise NotImplementedError(
NotImplementedError: Could not find signature for _fill_empty: <DataFrame, int>