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_selector_test.py

@@ -1,3 +1,5 @@
+from packaging.specifiers import SpecifierSet
+
 from cibuildwheel.util import BuildSelector
 
 
@@ -69,3 +71,36 @@ def test_build_braces():
     assert build_selector('cp37-manylinux1_x86_64')
     assert not build_selector('cp38-manylinux1_x86_64')
     assert not build_selector('cp39-manylinux1_x86_64')
+
+
+def test_build_limited_python():
+    build_selector = BuildSelector(build_config="*", skip_config="", requires_python=SpecifierSet(">=3.6"))
+
+    assert not build_selector('cp27-manylinux1_x86_64')
+    assert build_selector('cp36-manylinux1_x86_64')
+    assert build_selector('cp37-manylinux1_x86_64')
+    assert not build_selector('cp27-manylinux1_i686')
+    assert build_selector('cp36-manylinux1_i686')
+    assert build_selector('cp37-manylinux1_i686')
+    assert not build_selector('cp27-win32')
+    assert build_selector('cp36-win32')
+    assert build_selector('cp37-win32')
+    assert not build_selector('pp27-win32')
+    assert build_selector('pp36-win32')
+    assert build_selector('pp37-win32')
+
+
+def test_build_limited_python_partial():
+    build_selector = BuildSelector(build_config="*", skip_config="", requires_python=SpecifierSet(">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*"))
+
+    assert build_selector('cp27-manylinux1_x86_64')
+    assert not build_selector('cp35-manylinux1_x86_64')
+    assert build_selector('cp36-manylinux1_x86_64')
+
+
+def test_build_limited_python_patch():
+    build_selector = BuildSelector(build_config="*", skip_config="", requires_python=SpecifierSet(">=2.7.9"))
+
+    assert build_selector('cp27-manylinux1_x86_64')
+    assert build_selector('cp36-manylinux1_x86_64')
+    assert build_selector('cp37-manylinux1_x86_64')