only import a module if we use something from it

This prevents a bogus line such as `from types import ` from being
generated.

surprisingly this was not detected as a problem until a change
in isort! Formerly it would remove the bad line, but now it (correctly!)
errors

tools/extract_pyi.py

@@ -228,6 +228,7 @@ def convert_folder(top_level, stub_directory):
     imports, type_imports = extract_imports(tree)
     import_lines = ["from __future__ import annotations"]
     for type_module, used_types in type_imports.items():
+        if used_types:
             import_lines.append(f"from {type_module} import {', '.join(sorted(used_types))}")
     import_lines.extend(f"import {m}" for m in sorted(imports))
     import_body = "\n".join(import_lines)