Enhance python package and distribute wheels for multiple platforms

[macisamuele]

This PR:

• adds jsonschema_rs.__build__ which contains useful build information that will be relevant in case of support in the debugging of issues with the package
• ensures that the python library build is reproducible: this is "the opposite" of style: A rust library should not pin exact versions (use bounds instead) #90 :) . Cargo.lock file should be present if we're building an artifact that won't be used by rust and a Python library is one example of it.
• Removed bash scripts to build source and binary distributions. This is done in order to keep better support for windows developers (they might not have an easy access to bash) as well as we can leverage tox for dealing with Python virtual environments.
• Better usage of github-actions: use pypa/gh-action-pypi-publish to publish artifacts on pypi

NOTE: In order to obtain reproducible builds we need to ensure that python/Cargo.toml contains a published version and not a path.
Unfortunately this might limit the capability to run the python tests against the jsonschema rust library modified. In order to workaround this I've modified the setup.py in order to accept in input an environmental variable (UNRELEASED_JSONSCHEMA_PATH) which should point to the root of jsonschema-rs git repo. By doing so we ensure that:

• python build are reproducible at every commit
• we can test the effect of the rust changes on the python library without actually releasing the rust library yet (and this is directly used in the CI)

I'm fully aware that this PR, again, is not small and I'm truly sorry for it :)

This fixes: #110

cc: @Stranger6667

[Stranger6667]

Solid changes! Looks great to me :) Will issue 0.3.1 release shortly