Failures in teardown of temporary environment on Windows with dll in package

[jaraco]

While troubleshooting coherent-oss/coherent.deps#1, I ran into an error where on Windows, coh test will fail to exit cleanly if the tests imported a binary module (a DLL):

PermissionError: [WinError 5] Access is denied:
'C:\\Users\\jaraco\\AppData\\Local\\Temp\\pip-run-pdh6qyq1\\bson\\_cbson.cp312-win_amd64.pyd'

Here's what's happening:

• coherent.test uses pip-run to install the project and its test dependencies.
• in the same process, coherent.test launches pytest with those dependencies on the path.
• the tests run and in running those dependencies, a DLL is linked into the process, causing its file to be inaccessible (for write or delete).
• even after pytest has torn down, the Python process still has a handle on that module, so it cannot be deleted.

This may be yet another reason to launch pytest in a subprocess instead of in the same process that needs to clean up the pip-run environment.