Annotate ColumnAccessor._data labels as Hashable

[mroeschke]

Description

The motivating change here is that since we store a dictionary of columns in ColumnAccessor, the labels should be collections.abc.Hashable and therefore we can type methods that select by key with this annotation.

This led to a mypy-typing-validation cascade that made me type the output of def as_column(...) -> ColumnBase which also lead to typing validation in several other files.

Namely there no logic changes here.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

/merge