Envcast

In this release was fixed very sad mistake — in earlier release such dependencies as pytest, pylint, mypy was required for envcast to installation.
Now this things are no longer required to install envcast.

This is technical release mostly.

1. Now we can easily migrate tons of metafiles to trash and replace them with one solid and hard pyproject.toml
2. Pipenv now completely replaced with poetry
3. Added mypy check
4. Added python 3.10 for tests matrix

Happy usage!

Some new features and improvements:
* casting to None — now returns "None" suitable for desired type, not generic None. So if you desired type is str, now returns "" (empty string)
* added support of set and frozenset, bytes types
* improved test coverage
* added tests for complex type
* now more then 100 parametrized tests

Basic casting library + 100% test coverage. Check usage examples in Readme.
To setup package from source you need to do following steps:
* unpack
* run `pipenv install`
* then run `pipenv run flit install`