fix: add plain postgres alias

[betodealmeida]

SUMMARY

We added an alias so that postgres+psycopg2:// would continue to work after upgrading to SQLAlchemy 1.4 (where "postgres" should be "postgresql"), but forgot to add one for a plain postgres:// scheme.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

WIP

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 #21045 (459c589) into master (7445eab) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #21045      +/-   ##
==========================================
+ Coverage   66.24%   66.27%   +0.02%     
==========================================
  Files        1769     1769              
  Lines       67475    67475              
  Branches     7171     7171              
==========================================
+ Hits        44697    44716      +19     
+ Misses      20947    20928      -19     
  Partials     1831     1831              

Flag	Coverage Δ
hive	53.15% <ø> (ø)
mysql	80.94% <ø> (?)
postgres	80.99% <ø> (ø)
presto	53.05% <ø> (ø)
python	81.47% <ø> (+0.05%)	⬆️
sqlite	79.60% <ø> (ø)
unit	50.49% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/views/core.py	75.52% <0.00%> (+0.45%)	⬆️
superset/models/core.py	88.75% <0.00%> (+0.71%)	⬆️
superset/initialization/__init__.py	91.69% <0.00%> (+1.73%)	⬆️
superset/db_engine_specs/mysql.py	98.80% <0.00%> (+4.76%)	⬆️
superset/common/utils/dataframe_utils.py	95.23% <0.00%> (+4.76%)	⬆️

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