tools/gen-cpydiff: Skip Black fmt comments.

Since cpydiff is code used as documentation, there are cases where we may
want to use Black's `fmt: on/off/skip` comments to avoid automatic
formatting.  However, we don't want these comments to be distracting in the
generated documentation.

This rewrites the code to omit these comments when generating the docs.

Signed-off-by: David Lechner <david@pybricks.com>

tools/gen-cpydiff.py

@@ -85,6 +85,16 @@ def readfiles():
             class_, desc, cause, workaround, code = [
                 x.rstrip() for x in list(filter(None, re.split(SPLIT, text)))
             ]
+
+            # remove black `fmt: on/off/skip` comments
+            code = "".join(
+                # skip comments are inline, so we replace just the comment
+                re.sub(r"\s*# fmt: skip", "", x)
+                for x in code.splitlines(keepends=True)
+                # on/off comments are on their own line, so we omit the entire line
+                if not re.match(r"\s*# fmt: (on|off)\s*", x)
+            )
+
             output = Output(test, class_, desc, cause, workaround, code, "", "", "")
             files.append(output)
         except IndexError: