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/projectfiles_test.py

@@ -0,0 +1,142 @@
+from textwrap import dedent
+
+from cibuildwheel.projectfiles import get_requires_python_str, setup_py_python_requires
+
+
+def test_read_setup_py_simple(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = "1.23",
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) == "1.23"
+    assert get_requires_python_str(tmp_path) == "1.23"
+
+
+def test_read_setup_py_full(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            import setuptools
+
+            setuptools.randomfunc()
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = "1.24",
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) == "1.24"
+    assert get_requires_python_str(tmp_path) == "1.24"
+
+
+def test_read_setup_py_assign(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            REQUIRES = "3.21"
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = REQUIRES,
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) is None
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_setup_py_None(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            REQUIRES = None
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+                python_requires = None,
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) is None
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_setup_py_empty(tmp_path):
+    with open(tmp_path / "setup.py", "w") as f:
+        f.write(dedent("""
+            from setuptools import setup
+
+            REQUIRES = "3.21"
+
+            setuptools.setup(
+                name = "hello",
+                other = 23,
+                example = ["item", "other"],
+            )
+            """))
+
+    assert setup_py_python_requires(tmp_path.joinpath("setup.py").read_text()) is None
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_setup_cfg(tmp_path):
+    with open(tmp_path / "setup.cfg", "w") as f:
+        f.write(dedent("""
+            [options]
+            python_requires = 1.234
+            [metadata]
+            something = other
+            """))
+
+    assert get_requires_python_str(tmp_path) == "1.234"
+
+
+def test_read_setup_cfg_empty(tmp_path):
+    with open(tmp_path / "setup.cfg", "w") as f:
+        f.write(dedent("""
+            [options]
+            other = 1.234
+            [metadata]
+            something = other
+            """))
+
+    assert get_requires_python_str(tmp_path) is None
+
+
+def test_read_pyproject_toml(tmp_path):
+    with open(tmp_path / "pyproject.toml", "w") as f:
+        f.write(dedent("""
+            [project]
+            requires-python = "1.654"
+
+            [tool.cibuildwheel]
+            something = "other"
+            """))
+
+    assert get_requires_python_str(tmp_path) == "1.654"
+
+
+def test_read_pyproject_toml_empty(tmp_path):
+    with open(tmp_path / "pyproject.toml", "w") as f:
+        f.write(dedent("""
+            [project]
+            other = 1.234
+            """))
+
+    assert get_requires_python_str(tmp_path) is None