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

cibuildwheel/util.py

@@ -13,12 +13,15 @@ from typing import Dict, List, NamedTuple, Optional, Set
 import bracex
 import certifi
 import toml
+from packaging.specifiers import SpecifierSet
+from packaging.version import Version
 
 from .architecture import Architecture
 from .environment import ParsedEnvironment
 from .typing import PathOrStr, PlatformName
 
 resources_dir = Path(__file__).parent / 'resources'
+
 get_pip_script = resources_dir / 'get-pip.py'
 install_certifi_script = resources_dir / "install_certifi.py"
 
@@ -53,13 +56,25 @@ class IdentifierSelector:
     """
     This class holds a set of build/skip patterns. You call an instance with a
     build identifier, and it returns True if that identifier should be
-    included.
+    included. Only call this on valid identifiers, ones that have at least 2
+    numeric digits before the first dash.
     """
-    def __init__(self, *, build_config: str, skip_config: str):
+
+    def __init__(self, *, build_config: str, skip_config: str, requires_python: Optional[SpecifierSet] = None):
         self.build_patterns = build_config.split()
         self.skip_patterns = skip_config.split()
+        self.requires_python = requires_python
 
     def __call__(self, build_id: str) -> bool:
+        # Filter build selectors by python_requires if set
+        if self.requires_python is not None:
+            py_ver_str = build_id.split('-')[0]
+            major = int(py_ver_str[2])
+            minor = int(py_ver_str[3:])
+            version = Version(f"{major}.{minor}.99")
+            if not self.requires_python.contains(version):
+                return False
+
         build_patterns = itertools.chain.from_iterable(bracex.expand(p) for p in self.build_patterns)
         skip_patterns = itertools.chain.from_iterable(bracex.expand(p) for p in self.skip_patterns)
 
@@ -78,6 +93,8 @@ class BuildSelector(IdentifierSelector):
     pass
 
 
+# Note that requires-python is not needed for TestSelector, as you can't test
+# what you can't build.
 class TestSelector(IdentifierSelector):
     def __init__(self, *, skip_config: str):
         super().__init__(build_config="*", skip_config=skip_config)