Venvctrl

Thanks to mmeier162 for fixing nushell support.

This release modifies nushell activation script handling to work with both old and new versions. Existing users who were pinned to virtualenv<=20.11.2 are unaffected. Those wishing to use venvctrl with the latest virtualenv versions may now do so without needing to also disable nushell support.

Virtualenv 20.8.0 and above includes a new activate script for nu shell. This release add support for that new file.

A patch introduced in virtualenv 20.0.0 changed double quotes to single
quotes for the activate script variables that contain the embedded path
of the virtualenv. The existing tests were able to detect the regression
when run with the latest virtualenv. This patch modifies the regexes
used to detect the embedded path by checking for both kinds of quote
characters. Additionally, the output format of modified paths now uses
single quotes anywhere that virtualenv does so the outputs match.

- Add recursive .pth detection and path rewriting
- Add support xsh scripts
- Format project using black