Allow custom host for PyPI mirrors

[pilosus]

PyPI URL is hardcoded in the checker's code.
Given that people may be using PyPI mirrors, we need a CLI option to pass in a custom URL for JSON API

[pilosus]

It seems that no major Python repo servers support neither Warehouse JSON API, nor Simple API with JSON response as per PEP 691:

1. Sonatype Nexus - feature request
2. JFrog Artifactory - feature request
3. devpi - feature request