BUG/DEPR: Fix unwanted warnings from the Series(Index[datetime objects]) deprecation
See https://github.com/pandas-dev/pandas/pull/36697#discussion_r497786546 and https://github.com/pandas-dev/pandas/pull/37193#issuecomment-715228272

Due to the `Index.is_all_dates` + `Series(Index[datetime object])` deprecation warnings, there are several code examples that are not using this attribute or constructor method directly, that still cause this warning (from internal pandas code, without direct control over this by the user). 

Users should not see unrelated deprecation warnings from the internals of pandas that they can't do anything about themselves, so IMO we should either fix those for 1.2, or delay the deprecationg for 1.3