treat .tgz the same as .tar.gz (#7201)
## Summary
Fixes #7081
Treats source distribution `.tgz` the same as `.tar.gz` plans
## Test Plan
Quick Version
```bash
cd $(mktemp -d)
uv init
uv add --dev build
.venv/bin/python -m build -s .
mv -v dist/*tar.gz dist/"$(basename dist/*.tar.gz .tar.gz)".tgz
uv pip install dist/*.tgz
```
Can add a proper test to the branch if requested
