Ignore .mypy_cache in .gitignore

Mypy writes its own .gitignore into the folder; this keeps it ignored if
that file is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: PDowney

.gitignore

@@ -20,6 +20,7 @@ Thumbs.db
 .cache/
 .cspellcache
 .eslintcache
+.mypy_cache/
 .php-cs-fixer.cache
 .phpcs-cache
 .phpstan.cache