SNOW-3121122: Bare bones support for SQLA2.1

[Dev-iL]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes (?) SNOW-3121122: Meta: Support for SQLAlchemy 2.1 #652

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
   • I am adding new logging messages
   • I am adding new credentials
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

   • Added SQLA2.1 to CI.
   • compat.py
     • Added a IS_VERSION_21 flag for preserving backward compatibility with SQLA 1.4 and 2.0.
     • Import _ORMSelectCompileState (private in 2.1) and re-exported as ORMSelectCompileState.
   • base.py
     • SnowflakeORMSelectCompileState now inherits from compat.ORMSelectCompileState.
     • SnowflakeSelectState._setup_joins: ported the explicit_left pattern from SQLA 2.1's SelectState._setup_joins.
     • SnowflakeORMSelectCompileState._join_left_to_right: ported the explicit_left pattern from SQLA 2.1's _ORMSelectCompileState._join_left_to_right using the appropriate snowflake classes.
   • util.py
     • Guarded legacy _joined_from_info / string onclause code behind not IS_VERSION_21 (since removed in SQLA 2.1)
     • Synced augment_onclause logic with SQLA 2.1's upstream bug fix

[sfc-gh-jcieslak]

Hey @Dev-iL 👋
Thanks again for the contribution. We’re currently focused on a few ongoing topics, but we should be able to review it within the next few days. Apologies for the delay.