Update CI to use Python 3.7.

There's an open issue with setup-python action with Python 3.6 on Ubuntu.
For details, see actions/setup-python#544

.github/workflows/integ-test.yml

@@ -10,10 +10,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.6
+    - name: Set up Python 3.7
       uses: actions/setup-python@v2
       with:
-        python-version: 3.6
+        python-version: 3.7
     - name: Install Python dependencies
       run: pip install -e .[test,dev]
     - name: Run Integration Tests

.github/workflows/quality.yml

@@ -7,10 +7,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.6
+    - name: Set up Python 3.7
       uses: actions/setup-python@v2
       with:
-        python-version: 3.6
+        python-version: 3.7
     - name: Install Python dependencies
       run: pip install -e .[quality]
     - name: Run Quality check

.github/workflows/unit-test.yml

@@ -7,10 +7,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.6
+    - name: Set up Python 3.7
       uses: actions/setup-python@v2
       with:
-        python-version: 3.6
+        python-version: 3.7
     - name: Install Python dependencies
       run: pip install -e .[test,dev]
     - name: Run Unit Tests