[BUG] weird / flaky behavior in the Python CIs

[iscai-msft]

Running into flaky failures that I can't repro locally. The failures are different across the different matrices:

1. for linux 2.7 I had coverage errors like this (sdist and whl commands passed for mgmt, but then I got a ton of Coverage.py warnings for data plane and the whole command ended up failing)
2. for linux 3.8 I ran into "Visible uncombined .coverage files"
3. for windows 3.5 I ran into a permission error of "the process cannot access the file because it is being used by another process"

I've tried re-running, and some errors no longer occur. I've seen this behavior in three of my key vault PRs, but it seems like it's happening in pretty much all of the python pipelines (here's a failing core PR, thanks Charles!)

[iscai-msft]

@kristapratico added she's seeing these errors: https://teams.microsoft.com/l/message/19:[email protected]/1592843100575?tenantId=72f988bf-86f1-41af-91ab-2d7cd011db47&groupId=3e17dcb0-4257-4a30-b843-77f47f1d4121&parentMessageId=1592843100575&teamName=Azure%20SDK&channelName=Engineering%20System%20%F0%9F%9B%A0%EF%B8%8F&createdTime=1592843100575

[scbedd]

This ALL boils down to virtualenv updates that break us. Temporarily pinning to 20.0.23 to more fully understand why 20.0.25 doesn't work for us.