Fix popen_spawn when specifying a path on Windows which contains "\" separators

[eli-b]

I've opened an identical PR for pexpect: pexpect/pexpect#446

[kennethreitz]

what exactly does this do?

[kennethreitz]

everything is behaving as expected right now.

[eli-b]

This lets me run pipenv install --three --python=C:\Miniconda2\envs\python3\python.exe -d on Windows without getting WindowsError: [Error 2] The system cannot find the file specified. shlex.split() without posix=False treats backslashes as escapes instead of preserving them.

There's an open StackOverflow question about this issue in popen_expect.

I should have explained the problem this solves for me.

[uranusjr]

Is posix=(os.sep == '/') maybe a more pragmatic solution?

[techalchemy]

given that we are specifically interested in whether we are dealing with a posix compliant platform I would recommend posix=(os.name == 'posix')

[eli-b]

I just copied the test from earlier in the function where sys.platform == 'win32' is used. Testing for a POSIX system directly might be clearer.

[techalchemy]

I have a slight change to this over in #570 for reference