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/io/file_readinto.py

@@ -7,8 +7,8 @@ print(f.readinto(b))
 print(b)
 
 # readinto() on writable file
-f = open('io/data/file1', 'ab')
+f = open("io/data/file1", "ab")
 try:
     f.readinto(bytearray(4))
 except OSError:
-    print('OSError')
+    print("OSError")