Set default value for PYTHON_VERSION and always set python interpreter

.azure-pipelines/templates/jobs/extended-tests-jobs.yml

@@ -3,6 +3,8 @@ jobs:
     variables:
       - name: IMAGE_NAME
         value: ubuntu-18.04
+      - name: PYTHON_VERSION
+        value: 3.8
       - group: certbot-common
     strategy:
       matrix:

.azure-pipelines/templates/jobs/standard-tests-jobs.yml

@@ -1,5 +1,7 @@
 jobs:
   - job: test
+    variables:
+      PYTHON_VERSION: 3.8
     strategy:
       matrix:
         macos-py27:

.azure-pipelines/templates/steps/tox-steps.yml

@@ -21,7 +21,6 @@ steps:
     inputs:
       versionSpec: $(PYTHON_VERSION)
       addToPath: true
-    condition: ne(variables['PYTHON_VERSION'], '')
     # tools/pip_install.py is used to pin packages to a known working version
     # except in tests where the environment variable CERTBOT_NO_PIN is set.
     # virtualenv is listed here explicitly to make sure it is upgraded when