Update pytests action version and run sequentially (#1236)

@aerorahul and @guillaumevernieres noticed that some pytests were failing abruptly in the CI.
Manually booting them fixed the issue.  However, we want those tests to run automatically.

This PR executes the pytests for the different versions of python in the matrix sequentially.
It is hypothesized (not confirmed), that the pytest artifacts were being clobbered due to parallel execution.

This PR also updates the actions to a later version.
This eliminates certain deprecation warnings

.github/workflows/linters.yaml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v1
+        uses: actions/checkout@v3
         with:
           fetch-depth: 0
 

.github/workflows/pytests.yaml

@@ -6,12 +6,13 @@ jobs:
     runs-on: ubuntu-latest
     name: Install pygw and run tests with pytests
     strategy:
+      max-parallel: 1
       matrix:
         python: ["3.7", "3.8", "3.9", "3.10"]
 
     steps:
       - name: Setup Python
-        uses: actions/setup-python@v3
+        uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.python }}