Wagon

* Declare a dummy `venv` extras-require for compatibility with widely-deployed Cloudify CLI 7.0.2

* `distro` package as optional dependency.
* Add static files support. User can add,
list or get static files from the Wagon.
* Drop Python 2 support.
* Switch `virtualenv` to stdlib `venv`.
* Add `python_requires` information to METADATA.

* Update dependencies
* Add Python 3.7, 3.8, 3.9, 3.10 classifiers
* AppVeyor configuration upgrade
* Add CircleCI configuration

* Pass supported platform to the create function , and handle new wheel naming convention in case of manylinux.

* Sort wheel list during platform naming to ensure that wagons are consistently labelled from build to build.

* When a wagon would be tagged "manylinux", tag it with linux_x86_64 instead.
This is done to allow supporting Cloudify 5.1 (and earlier), which doesn't
handle manylinux wagons correctly.
* `wagon create` now supports a `--supported-platform` flag, allowing the
supported platform field to be overridden when creating a wagon.