Upgrade Click-Completion and apply Shellingham for shell detection

[uranusjr]

Fix #2352. I want to do this without manually patching Click-Completion if possible.

https://github.com/pypa/pipenv/projects/3

[uranusjr]

This introduces an additional environment variable PIPENV_SHELL. If set, shell detection is skipped completely. The old SHELL introspection is kept as a fallback if detection fails.

A run down of how shell detection works now:

1. If PIPENV_SHELL is set, use it.
2. Detect surrounding shell…
   1. (Only on POSIX) The surrounding shell is the login shell. Use the value of SHELL.
   2. Use the detected shell executable.
3. Detection failed. Check if SHELL is set; if so, use it.
4. Prompt the user to set PIPENV_SHELL explicitly.

[techalchemy]

looks good to me

[frostming]

Does this PR work under CPython only?(see the 'ctypes' imports in Shellingham)
I guess the classifier should be removed from setup.py. In fact PyPy is not tested at all.

[techalchemy]

We rely heavily on ctypes on window and have done so for some time. We use this as a means of shell detection for spawning subshells. This isn’t new, although I’m guessing the complete lack of issues on the topic is a good indicator that things are ok

[uranusjr]

ctypes does exist on PyPy. It is a wrapper of CFFI, and is slow, but works.