tests: Format all Python code with black, except tests in basics subdir.

This adds the Python files in the tests/ directory to be formatted with
./tools/codeformat.py.  The basics/ subdirectory is excluded for now so we
aren't changing too much at once.

In a few places `# fmt: off`/`# fmt: on` was used where the code had
special formatting for readability or where the test was actually testing
the specific formatting.

tests/unicode/unicode_str_format.py

@@ -1,4 +1,4 @@
 # test handling of unicode chars in format strings
 
-print('α'.format())
-print('{α}'.format(α=1))
+print("α".format())
+print("{α}".format(α=1))