[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. 🤖🎉

[emmanuel-ferdman]

Hi @srittau @donbarbos, sorry for bumping, but could you please review this PR when you get the chance?

[srittau]

It seems that ultimately, os.execvpe is called (via ptyprocess):

https://github.com/pexpect/ptyprocess/blob/97f068c80952c702873d801d2c8b262b79685d38/ptyprocess/ptyprocess.py#L298C24-L298C31

In that case it's probably best to just use os._ExecEnv as the annotation. We might tighten that annotation in the future, since it seems overly broad. (Mapping is quite problematic in that regard.)