As noted by @eryksun in [1] and [2], using _cleanup and _active(in
__del__) is not necessary on Windows, since:

> Unlike Unix, a process in Windows doesn't have to be waited on by
> its parent to avoid a zombie. Keeping the handle open will actually
> create a zombie until the next _cleanup() call, which may be never
> if Popen() isn't called again.

This patch simply defines `subprocess._active` as `None`, for which we already
have the proper logic in place in `subprocess.Popen.__del__`, that prevents it
from trying to append the process to the `_active`. This patch also defines
`subprocess._cleanup` as a noop for Windows.

[1] https://bugs.python.org/issue37380#msg346333
[2] https://bugs.python.org/issue36067#msg336262

Signed-off-by: Ruslan Kuprieiev <ruslan@iterative.ai>

Fix an unintended ValueError from :func:`subprocess.run` when checking for
conflicting `input` and `stdin` or `capture_output` and `stdout` or `stderr` args
when they were explicitly provided but with `None` values within a passed in
`**kwargs` dict rather than as passed directly by name.

Classes that define __str__ the same as __repr__ can
just inherit it from object.

Close pipes/fds in subprocess by using ExitStack.

"In case of premature failure on X.Close() or os.close(X) the remaining pipes/fds will remain "open". Perhaps it makes sense to use contextlib.ExitStack."
- Rationale: https://github.com/python/cpython/pull/11575#discussion_r250288394

* subprocess.Popen can now also use os.posix_spawn() with pipes,
  but only if pipe file descriptors are greater than 2.
* Fix Popen._posix_spawn(): set '_child_created' attribute to True.
* Add Popen._close_pipe_fds() helper function to factorize the code.

This reverts commit 07858894689047c77f9c12ddc061d30681368d19.

The subprocess module can now use the os.posix_spawnp() function,
if it is available, to locate the program in the PATH.