chore(#39): install python version specified

original yaml file was not installing specified version in action
matrix.
added action to install specified version for python.

bumping python 3.6.2 to 3.6.7 as following are the versions valid for
the new action used.
https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

.github/workflows/main.yml

@@ -19,11 +19,15 @@ jobs:
     strategy:
       matrix:
         python-version:
-         - "3.6.2"
+         - "3.6"
          - "3.10"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
+      - name: Install Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
       - uses: psf/black@stable
       - name: Build
         run: |
@@ -42,7 +46,7 @@ jobs:
     strategy:
       matrix:
         python-version:
-         - "3.6.2"
+         - "3.6"
          - "3.10"
     services:
       mysql:
@@ -57,9 +61,13 @@ jobs:
           --health-interval 5s
           --health-timeout 2s
           --health-retries 3
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
+      - name: Install Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
       - uses: psf/black@stable
       - name: Build
         run: |
@@ -77,7 +85,7 @@ jobs:
     strategy:
       matrix:
         python-version:
-         - "3.6.2"
+         - "3.6"
          - "3.10"
     services:
       postgres:
@@ -92,9 +100,13 @@ jobs:
           --health-interval 5s
           --health-timeout 2s
           --health-retries 3
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
+      - name: Install Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
       - uses: psf/black@stable
       - name: Build
         run: |