feat: requires-python (#536)

* feat: requires-python

refactor: updating to current proposal

fix: add tests and fix a few issues

docs: Update mostly from @joerick

docs: update README from readthedocs

* refactor: pull out config file reading

* fix: always assume highest Python patch version

* refactor: try new design

* refactor: function and bump MyPy to 0.800

* fix: tighten AST to call only, add docs

* fix: address review points from @joerick

unit_test/build_ids_test.py

@@ -1,11 +1,8 @@
-import pytest
 import toml
-
-from cibuildwheel.util import resources_dir
-
-Version = pytest.importorskip("packaging.version").Version
+from packaging.version import Version
 
 from cibuildwheel.extra import InlineArrayDictEncoder  # noqa: E402
+from cibuildwheel.util import resources_dir
 
 
 def test_compare_configs():