Types: Unlock supporting timezone-aware `DateTime` fields

[amotl]

About

Timezone-aware SQLAlchemy DateTime fields apparently are supported already. This patch just removes a corresponding assertion which prevented them from actually being used.

References

Originally, this is coming from a monkeypatch to meltano-target-cratedb. Most probably, it is also related to those requests:

• crate/sqlalchemy-cratedb#92
• crate/crate-python#626

Other than this, that other patch is also related:

• GH-25

Backlog

• [x] Software tests.
• [x] Changelog item.
• [x] Check if documentation needs to be updated.