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/projectfiles.py

@@ -0,0 +1,79 @@
+import ast
+import sys
+from configparser import ConfigParser
+from pathlib import Path
+from typing import Any, Optional
+
+import toml
+
+if sys.version_info < (3, 8):
+    Constant = ast.Str
+
+    def get_constant(x: ast.Str) -> str:
+        return x.s
+
+
+else:
+    Constant = ast.Constant
+
+    def get_constant(x: ast.Constant) -> Any:
+        return x.value
+
+
+class Analyzer(ast.NodeVisitor):
+    def __init__(self) -> None:
+        self.requires_python: Optional[str] = None
+
+    def visit(self, content: ast.AST) -> None:
+        for node in ast.walk(content):
+            for child in ast.iter_child_nodes(node):
+                child.parent = node  # type: ignore
+        super().visit(content)
+
+    def visit_keyword(self, node: ast.keyword) -> None:
+        self.generic_visit(node)
+        if node.arg == "python_requires":
+            # Must not be nested in an if or other structure
+            # This will be Module -> Expr -> Call -> keyword
+            if (
+                not hasattr(node.parent.parent.parent, "parent")  # type: ignore
+                and isinstance(node.value, Constant)
+            ):
+                self.requires_python = get_constant(node.value)
+
+
+def setup_py_python_requires(content: str) -> Optional[str]:
+    try:
+        tree = ast.parse(content)
+        analyzer = Analyzer()
+        analyzer.visit(tree)
+        return analyzer.requires_python or None
+    except Exception:
+        return None
+
+
+def get_requires_python_str(package_dir: Path) -> Optional[str]:
+    "Return the python requires string from the most canonical source available, or None"
+
+    # Read in from pyproject.toml:project.requires-python
+    try:
+        info = toml.load(package_dir / 'pyproject.toml')
+        return str(info['project']['requires-python'])
+    except (FileNotFoundError, KeyError, IndexError, TypeError):
+        pass
+
+    # Read in from setup.cfg:options.python_requires
+    try:
+        config = ConfigParser()
+        config.read(package_dir / 'setup.cfg')
+        return str(config['options']['python_requires'])
+    except (FileNotFoundError, KeyError, IndexError, TypeError):
+        pass
+
+    try:
+        with open(package_dir / 'setup.py') as f:
+            return setup_py_python_requires(f.read())
+    except FileNotFoundError:
+        pass
+
+    return None