ci: limit parallel jobs to avoid tidal rate limit

Author: JuniorJPDJ

.github/workflows/tests.yml

@@ -12,6 +12,7 @@ jobs:
     name: PyTest
 
     strategy:
+      max-parallel: 2
       matrix:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
         python-version: [ '3.7', '3.8', '3.9', 'pypy-3.7' ]