fix: SQLite dttm ISO format

[michael-s-molina]

SUMMARY

Fix SQLite dttm ISO format to use microseconds as the timespec. This resolves a bug when comparing dttm values with DATETIME and TIMESTAMP columns on SQLite.

TESTING INSTRUCTIONS

There's a unit test for this function. All tests should pass.

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 #22128 (d51b0b6) into master (6f6cb18) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #22128      +/-   ##
==========================================
- Coverage   66.98%   66.93%   -0.06%     
==========================================
  Files        1832     1832              
  Lines       69918    69918              
  Branches     7570     7570              
==========================================
- Hits        46838    46802      -36     
- Misses      21122    21158      +36     
  Partials     1958     1958              

Flag	Coverage Δ
hive	52.60% <0.00%> (ø)
mysql	?
postgres	78.20% <0.00%> (-0.02%)	⬇️
presto	52.49% <0.00%> (ø)
python	81.27% <100.00%> (-0.11%)	⬇️
sqlite	?
unit	50.85% <100.00%> (ø)

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

Impacted Files	Coverage Δ
superset/db_engine_specs/sqlite.py	89.28% <100.00%> (-7.15%)	⬇️
superset/common/utils/dataframe_utils.py	90.47% <0.00%> (-4.77%)	⬇️
superset/db_engine_specs/mysql.py	94.04% <0.00%> (-4.77%)	⬇️
superset/reports/commands/log_prune.py	85.71% <0.00%> (-3.58%)	⬇️
superset/utils/celery.py	86.20% <0.00%> (-3.45%)	⬇️
superset/connectors/sqla/utils.py	87.37% <0.00%> (-1.95%)	⬇️
superset/result_set.py	96.37% <0.00%> (-1.45%)	⬇️
superset/commands/importers/v1/utils.py	92.20% <0.00%> (-1.30%)	⬇️
superset/views/core.py	75.05% <0.00%> (-1.06%)	⬇️
... and 4 more

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[zhaoyongjie]

I remember that this line was fixed by PR because the SQLite manual said that The datetime() function returns the date and time as text in their same formats: YYYY-MM-DD HH:MM:SS.(link)

So, please check out the previous PR and prevent introducing a regression. Thanks.

[michael-s-molina]

Closing this in favor of #22170.