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/thread/mutate_set.py

@@ -21,6 +21,7 @@ def th(n, lo, hi):
         global n_finished
         n_finished += 1
 
+
 lock = _thread.allocate_lock()
 n_thread = 4
 n_finished = 0