.azurepiplines/pr-gate-steps.yml: Update python to 3.8.x for ci build

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2617
Update edk2 build and test ci to use Python 3.8.x

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>

.azurepipelines/templates/pr-gate-steps.yml

@@ -20,7 +20,7 @@ steps:
 
 - task: UsePythonVersion@0
   inputs:
-    versionSpec: '3.7.x'
+    versionSpec: '3.8.x'
     architecture: 'x64'
 
 - script: pip install -r pip-requirements.txt --upgrade