Support sqlalchemy 1.4

[cwegrzyn]

sqlalchemy 1.4 has some breaking changes. At the very least, we need to remove all uses of contextual_connect (which appears to be safe since we don't appear to use threadlocal anywhere).

Once sqlalchemy is supported in core records-mover, we should make sure that we only lift the restriction once all sqlalchemy packages have updated their own version constraints to account for their own support. (For example, sqlalchemy bigquery support does not currently support 1.4 but they have updated their constraints to reflect this)

[cwegrzyn]

Looks like bigquery sqlalchemy 1.4 support is close: googleapis/python-bigquery-sqlalchemy#177

[austinweisgrau]

bigquery compatibility has been achieved since just a few days after Chris's last message above, see: https://github.com/googleapis/python-bigquery-sqlalchemy/releases/tag/v0.8.0

[ryantimjohn]

Completed in #208