support virtual environments

[woutervh]

Currently the configuration is globally available for all virtual envs for a specific user:

~/.pipupgrade/config.ini

1. This violates the goal of virtual or isolated environments.
2. This also pollutes the $HOME,
   modern location should be ~/.config/pipupgrade/ according to XDG_CONFIG_DIRS
   (pip itself also move to ~/.config/pip/)

I propose to support an extra environment-variable:

PIPUPGRADE_CONFIG=/etc/pipupgrade.ini

[achillesrasquinha]

Good call!

[woutervh]

this is how pip itself solves it
https://pip.pypa.io/en/stable/user_guide/#config-file

they support PIP_CONFIG_FILE, but also a list of default location to search in.

[achillesrasquinha]

Done!