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> Maybe use SetEnvironmentVariable() on Windows? 
`_wputenv` keeps Python's copy of the environment block in sync with CRT's copy, which in turn calls `SetEnvironmentVariableW` to sync with process environment block. The CRT's copy of the environment excludes and disallows variable names that start with "=". Because the interpreter initializes `os.environ` from the CRT environment, Python has never included or allowed 'hidden' variables such as "=C:". 
Switching to calling `SetEnvironmentVariableW` directly won't affect `os.system` and `os.spawnv[e]`, since the CRT calls `CreateProcessW` without overriding the process environment. However, have you considered the consequences for extension modules and embedding applications that use the CRT environment (e.g. `getenv`, `_wgetenv`) if the interpreter stops syncing with it? 
The concern that `[_w]putenv` doesn't copy the string does not apply to Windows.