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_subscr.py

@@ -1,4 +1,4 @@
-a = '¢пр'
+a = "¢пр"
 
 print(a[0], a[0:1])
 print(a[1], a[1:2])