BUG: DatetimeIndex with time as key
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
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
ENH: Adds function to generate_bq_schema