CI: Update build workflow to remove old mbed-cli workarounds.

Author: microbit-carlos

.github/workflows/build.yml

@@ -19,11 +19,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: '3.6'
-      # Some dependencies used by the old Mbed OS version depend on pip < v10
-      # & pip v9 struggles to install new version of cryptography (which uses rust)
-      - run: pip install pip==9.0.3 cryptography==3.3.2
-      # Old version of mbedos can have dependency clashes with newer mbed-cli
+          python-version: '3.7'
       - run: pip install mbed-cli
       - run: mbed config root .
       - run: mbed deploy