Currently, os.putenv() is always implemented with putenv(). The problem is that putenv(str) puts directly the string into the environment, the string is not copied. So Python has to keep track of this memory.

In Python 3.9, this string is now cleared at Python exit, without unsetting the environment variable which cause bpo-39395 crash.

I propose to implement os.putenv() with setenv() if available, which avoids bpo-39395 on platforms providing setenv().