lint: Add test for package readme syntax errors (#492)

Add a test to ensure readmes render properly

Also adds README.rst for testutil package to pass new test.

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

scripts/check_for_valid_readme.py

@@ -0,0 +1,51 @@
+"""Test script to check given paths for valid README.rst files."""
+import argparse
+import sys
+from pathlib import Path
+
+import readme_renderer.rst
+
+
+def is_valid_rst(path):
+    """Checks if RST can be rendered on PyPI."""
+    with open(path) as readme_file:
+        markup = readme_file.read()
+    return readme_renderer.rst.render(markup) is not None
+
+
+def parse_args():
+    parser = argparse.ArgumentParser(
+        description="Checks README.rst file in path for syntax errors."
+    )
+    parser.add_argument(
+        "paths", nargs="+", help="paths containing a README.rst to test"
+    )
+    parser.add_argument("-v", "--verbose", action="store_true")
+    return parser.parse_args()
+
+
+def main():
+    args = parse_args()
+    error = False
+
+    for path in map(Path, args.paths):
+        readme = path / "README.rst"
+        try:
+            if not is_valid_rst(readme):
+                error = True
+                print("FAILED: RST syntax errors in", readme)
+                continue
+        except FileNotFoundError:
+            error = True
+            print("FAILED: README.rst not found in", path)
+            continue
+        if args.verbose:
+            print("PASSED:", readme)
+
+    if error:
+        sys.exit(1)
+    print("All clear.")
+
+
+if __name__ == "__main__":
+    main()