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/math_fun_intbig.py

@@ -8,4 +8,4 @@ except ImportError:
 
 for fun in (math.ceil, math.floor, math.trunc):
     for x in (-1e25, 1e25):
-        print('%.3g' % fun(x))
+        print("%.3g" % fun(x))