BUG: DatetimeIndex with time as key
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 #8925 from hkleynhans/fix/8514_option_context_with

BUG: Option context applies on __enter__
Merge pull request #8907 from behzadnouri/time-slice

BUG: DatetimeIndex with time object as key
BUG: fixed chunksize guessed to 0 (py3 only). #8621