> Modifying this value could have a real impact on the rest of the process, so we should be very careful to undo it regardless of test result. (Modifying HOME is not a as big a deal since, as you point out, it's not "real" ;) )

Agreed, though I'd like to write it off as an existing problem (if it is a problem, it looks to me like the tests are doing proper environment cleanup).  If they are broken, they'd already be leaking environment variables on posix since these tests run on both

Auditing the tests I modified:

- test_netrc: uses `with support.EnvironmentVarGuard() as environ:` (OK!)
- test_ntpath: uses `with support.EnvironmentVarGuard() as env:` (OK!)
- test_dist: `MetadataTestCase` extends `support.EnvironGuard` (OK!)
- test_config: `BasePyPIRCCommandTestCase` extends `support.EnvironGuard` (OK!)

so actually, looks like this is already good to go on that front!

Definitely agree on using the true value in the long term -- I'd like to defer that until that's actually happening though as it'll be a much more involved patch!