DOC: specify return type in to_datetime
CLN: move import to top of file

For consistency with [PEP8][1]:

    Imports are always put at the top of the file, just after any module
    comments and docstrings, and before module globals and constants.

[1]: https://www.python.org/dev/peps/pep-0008#id17
Merge pull request #8922 from aevri/refactor/move_import

CLN: move import to top of file
BUG: Option context applies on __enter__

Option context no longer overrides options when used outside a `with`
statement.

Added test TestConfig.test_option_config_scope

Closes #8514
Merge pull request #8921 from jorisvandenbossche/docstring-to_datetime

DOC: specify return type in to_datetime
Merge pull request #8925 from hkleynhans/fix/8514_option_context_with

BUG: Option context applies on __enter__
BUG: DatetimeIndex with time as key
TST: Moved test functions to module level. #8023
TST: changed *args, **kwargs to fixed arguments. #8023
TST: Moved rest of the test functions to module level. #8023
TST: patch all module level assert functions to TestCase. #8023
CLN: Cleanup TestCase assert functions pathing #8023