fix(postgres): remove redundant tz factory

[villebro]

SUMMARY

A recent PR #17290 bumped Psycopg2 from 2.8.5 to 2.9.1, which features many changes to how timezones are handled: https://www.psycopg.org/articles/2021/06/16/psycopg-29-released/. Most notable is the change replacing the internal FixedOffsetTimezone class with the regular datetime.timezone when creating datetime objects, which is the "normal behavior" when working with datetimes. As a consequence, the constructor call to cursor.tzinfo_factory previously received the offset in minutes, while it now receives it as a datetime.timedelta object. As the custom FixedOffsetTimezone was only needed to make the timezone serializable by PyArrow, we can now remove it, as datetime objects containing a timezone tzinfo are serializable by PyArrow:

>>> import pyarrow as pa
>>> from datetime import datetime, timezone
>>> buf = pa.serialize(datetime(year=2021, month=1, day=1, tzinfo=timezone.utc)).to_buffer()
sys:1: FutureWarning: 'pyarrow.serialize' is deprecated as of 2.0.0 and will be removed in a future version. Use pickle or the pyarrow IPC functionality instead.
>>> pa.deserialize(buf)
sys:1: FutureWarning: 'pyarrow.deserialize' is deprecated as of 2.0.0 and will be removed in a future version. Use pickle or the pyarrow IPC functionality instead.
datetime.datetime(2021, 12, 10, 0, 0)

AFTER

Now running the query SELECT TO_TIMESTAMP('2019-12-10 01:00:00.000000', 'YYYY-MM-DD HH24:MI:SS.US') works just fine:

BEFORE

Previously it failed with the following error:

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #17713 (68f47f2) into master (8e69b2d) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17713      +/-   ##
==========================================
- Coverage   68.13%   68.13%   -0.01%     
==========================================
  Files        1653     1653              
  Lines       66375    66371       -4     
  Branches     7120     7120              
==========================================
- Hits        45226    45220       -6     
- Misses      19252    19254       +2     
  Partials     1897     1897              

Flag	Coverage Δ
hive	81.78% <ø> (-0.01%)	⬇️
mysql	82.19% <ø> (-0.01%)	⬇️
postgres	82.20% <ø> (-0.01%)	⬇️
presto	82.06% <ø> (-0.01%)	⬇️
python	82.68% <ø> (-0.01%)	⬇️
sqlite	81.88% <ø> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
superset/db_engine_specs/postgres.py	97.27% <ø> (-0.10%)	⬇️
superset/db_engine_specs/presto.py	89.97% <0.00%> (-0.42%)	⬇️

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