type DatetimeIndex.indexer_at_time and DatetimeIndex.indexer_between_time

[MarcoGorelli]

This PR: adds annotations for:

• adds annotations for DatetimeIndex.indexer_at_time
• adds annotations for DatetimeIndex.indexer_between_time
• adds annotations for DatetimeIndex.snap
• removes DatetimeIndex.slice_indexer, as it can just inherit from Index.slice_indexer (there's no datetime-index-specific docs for this method)
• removes DatetimeIndex.__reduce__ (not documented)
• moves existing DatetimeIndex tests from tests/indexes/test_indexes.py to a new tests/indexes/test_datetime_index.py file
• adds new tests for DatetimeIndex.indexer_at_time and DatetimeIndex.indexer_between_time to the test_datetime_index file

[cmp0xff]

Thank you @MarcoGorelli !