Azure: Allow distros to install Python they want

The platforms may have different Pythons.
But due to [0] the Python installed via the 'UsePythonVersion@0'
task should be compatible with the container's 'libpythonxx.so'.
'AZURE_PYTHON_VERSION' platform variable is introduced to cover
this. So, if your distro has Python3.8, set the mentioned variable
to '3.8', later, this version will be installed by the
'UsePythonVersion@0' Azure task for 'WebUI_Unit_Tests' and 'Tox'
jobs.

To allow tox to run any Python3 environment the 'py3' one is used.
'py3' is the well-known Tox's environment, which utilizes 'python3'
executable.

[0]: https://github.com/microsoft/azure-pipelines-tasks/issues/11070

Fixes: https://pagure.io/freeipa/issue/8254
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Alexander Bokovoy <abbra@users.noreply.github.com>

ipatests/azure/azure-pipelines.yml

@@ -93,7 +93,7 @@ jobs:
     - template: templates/${{ variables.PREPARE_BUILD_TEMPLATE }}
     - task: UsePythonVersion@0
       inputs:
-        versionSpec: 3.7
+        versionSpec: ${{ variables.AZURE_PYTHON_VERSION }}
         architecture: x64
     - template: templates/${{ variables.PREPARE_TOX_TEMPLATE }}
     - script: |
@@ -102,7 +102,7 @@ jobs:
         export LANG=en_US.utf8
         export LC_CTYPE=en_US.utf8
         locale
-        $(TOX_COMMAND) -e py37,pypi,pylint3
+        $(TOX_COMMAND) -e py3,pypi,pylint3
       displayName: Tox
     - task: PublishTestResults@2
       inputs:
@@ -120,7 +120,7 @@ jobs:
     - template: templates/${{ variables.PREPARE_BUILD_TEMPLATE }}
     - task: UsePythonVersion@0
       inputs:
-        versionSpec: 3.7
+        versionSpec: ${{ variables.AZURE_PYTHON_VERSION }}
         architecture: x64
     - template: templates/${{ variables.PREPARE_WEBUI_TEMPLATE }}
     - script: |

ipatests/azure/templates/variables-fedora.yml

@@ -15,3 +15,6 @@ variables:
   PREPARE_WEBUI_TEMPLATE: ${{ format('prepare-webui-{0}.yml', variables.IPA_PLATFORM) }}
 
   TOX_COMMAND: tox
+
+  # Python version for UsePythonVersion@0 task
+  AZURE_PYTHON_VERSION: '3.7'

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion=2.3.1
-envlist=py36,py37,pylint3,pypi
+envlist=py36,py37,py38,pylint3,pypi
 skip_missing_interpreters=true
 skipsdist=true