Quick script to test for import loops

Since `test-docs.sh` was intermittently finding an import loop while I
was writing the previous commit, but only by chance, when it happened to
run the documentation in some particular order.

Author: brandon-rhodes

design/test_import_loops.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+
+import subprocess
+import sys
+from pathlib import Path
+
+def main():
+    my_path = Path(sys.argv[0])
+    design_dir = my_path.parent
+    project_dir = design_dir.parent
+    skyfield_dir = project_dir / 'skyfield'
+
+    for path in sorted(skyfield_dir.glob('*.py')):
+        module_name = path.stem
+        command = ['python3', '-c', 'import skyfield.' + module_name]
+        print(module_name)
+        try:
+            subprocess.check_call(command)
+        except subprocess.CalledProcessError:
+            exit(1)
+
+if __name__ == '__main__':
+    main()