GH1432 Partial resolution for some TODOs

[loicdiridollou]

• Closes Issues from TODOs in timestamps.pyi, timedeltas.pyi and orc.pyi #1432
• Tests added: Please use assert_type() to assert the type of any return value

Resolution for:

• read_orc (although there is no docs or tests in pandas repo to properly test that)
• Timestamp - datetime64 (There is the typing to the rsub but mypy sees the wrong type)
• tz_localize returning NaT (edge case of changing time with DST)