fix: allow datasource imports on Windows

[BTatlock]

SUMMARY

No sources are added when importing zip files on Windows, as file paths are parsed using pathlib.PureWindowsPath. Schemas are defined using prefixes with trailing slashes, therefore file paths in zipfiles should instead be treated as pathlib.PurePosixPaths.

Although Superset does not have official Windows support, this change fixes imports without changing the behaviour on Unix, since instantiating pathlib.Path creates a pathlib.PurePosixPath.

TESTING INSTRUCTIONS

Import zipfile (e.g. superset import_dashboards -p dashboard_export.zip) on WIndows

[github-actions]

Congrats on making your first PR and thank you for contributing to Superset! 🎉 ❤️
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[rusackas]

Thank you for the contribution! You're right, that while we don't support Windows, it never hurts to improve the experience for those who wish to go down that road. I'm pinging @betodealmeida and @eschutho for review since they're close to the feature, but also pinging @jinghua-qa in case there are any concerns with supporting this from a QA standpoint.

[betodealmeida]

This is great, thanks! And #til!

[codecov]

Codecov Report

Merging #22264 (9716a84) into master (fb3e717) will increase coverage by 1.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #22264      +/-   ##
==========================================
+ Coverage   65.54%   66.84%   +1.29%     
==========================================
  Files        1841     1841              
  Lines       70220    70220              
  Branches     7670     7670              
==========================================
+ Hits        46027    46936     +909     
+ Misses      22211    21302     -909     
  Partials     1982     1982              

Flag	Coverage Δ
hive	52.56% <33.33%> (ø)
mysql	78.07% <100.00%> (ø)
postgres	78.14% <100.00%> (ø)
presto	52.45% <33.33%> (ø)
python	81.33% <100.00%> (+2.71%)	⬆️
sqlite	76.59% <100.00%> (ø)
unit	50.87% <100.00%> (?)

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

Impacted Files	Coverage Δ
superset/commands/importers/v1/utils.py	93.50% <100.00%> (ø)
superset/databases/api.py	93.53% <0.00%> (+0.30%)	⬆️
superset/sql_lab.py	82.12% <0.00%> (+0.38%)	⬆️
superset/views/base_api.py	98.44% <0.00%> (+0.38%)	⬆️
superset/db_engine_specs/presto.py	88.22% <0.00%> (+0.41%)	⬆️
superset/db_engine_specs/__init__.py	85.71% <0.00%> (+1.19%)	⬆️
superset/datasets/schemas.py	97.40% <0.00%> (+1.29%)	⬆️
superset/commands/exceptions.py	93.84% <0.00%> (+1.53%)	⬆️
superset/initialization/__init__.py	90.96% <0.00%> (+2.00%)	⬆️
superset/utils/log.py	93.91% <0.00%> (+2.02%)	⬆️
... and 53 more

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