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/float/string_format_modulo3.py

@@ -1,3 +1,3 @@
 # uPy and CPython outputs differ for the following
-print("%.1g" % -9.9) # round up 'g' with '-' sign
-print("%.2g" % 99.9) # round up
+print("%.1g" % -9.9)  # round up 'g' with '-' sign
+print("%.2g" % 99.9)  # round up