Don't cancel all jobs in CI if one job fails

This will make it easier to see in python-attrs#384 which tests (if any) are actually failing on which Python versions -- currently the codecov failures are just leading to all jobs being cancelled in CI

Author: AlexWaygood

.github/workflows/main.yml

@@ -18,6 +18,7 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "pypy-3.8"]
+      fail-fast: false
 
     steps:
       - uses: "actions/checkout@v2"