[pexpect] Accept dict for env parameter, not just os.environ

[emmanuel-ferdman]

PR Summary

The env parameter in pexpect was typed as os._Environ[str], which only accepts os.environ directly. This broke common patterns like passing {"PATH": "/usr/bin"} or os.environ.copy(). The fix changes the type to Mapping[str, str] (or subprocess._ENV for PopenSpawn) to match what the code actually accepts. PopenSpawn passes env to subprocess.Popen, while spawn/pxssh/run pass it to both utils.which() (which does env.get('PATH') and requires string keys/values) and ptyprocess. All of these work fine with any dict-like mapping, not just the special os.environ object.

Closes #14915

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉