Run mypy_primer in all pull requests (#7804)

Author: Akuli

.github/workflows/mypy_primer.yml

@@ -3,10 +3,6 @@ name: Run mypy_primer
 on:
   # Only run on PR, since we diff against master
   pull_request:
-    # Since modular typeshed, mypy primer doesn't check third party stubs. Till that is changed,
-    # only run it on stdlib changes to avoid false confidence and wasted CI minutes.
-    paths:
-    - 'stdlib/**'
 
 jobs:
   mypy_primer: