Fix __pycache__ in .gitignore. NFC (#24048)

We were ignoring all `.pyc` files which is normally enough to also
ignore all `__pycache__` directories but I've seen some machines where
`__pycache__` also contains files ending in `.er`.

Author: sbc100

.gitignore

@@ -10,7 +10,7 @@
 *.swp
 
 # Compiled python files
-*.pyc
+__pycache__
 
 # Default emscripten cache
 /cache