Make python 3.7 default in CI and remove duplicate job (#3636)

Since we've deprecated python 3.5 having 3.5 be the first round of
testing performed in CI feels out of place. This commit makes the
default jobs run in the first stage of the CI pipeline in azure all use
python 3.7. We still need to verify that 3.5 works and still gate on the
3.5 job passing. But, it doesn't get as much attention now that it's
deprecated. At the same time this commit takes the opportunity to remove
the duplicated python 3.8 job in the travis config. This was added
originally because travis had 3.8 support well before azure pipelines
but was never removed after azure added their 3.8 support.

Co-authored-by: Kevin Krsulich <kevin@krsulich.net>

.travis.yml

@@ -62,10 +62,6 @@ jobs:
         - pip install diff-cover || true
         - diff-cover --compare-branch master coverage.xml || true
 
-    - name: Python 3.8 Tests
-      <<: *stage_linux
-      python: 3.8
-
     # Randomized testing
     - name: Randomized tests
       <<: *stage_linux

azure-pipelines.yml

@@ -122,8 +122,8 @@ stages:
       condition: not(startsWith(variables['Build.SourceBranch'], 'refs/tags'))
       strategy:
         matrix:
-          Python35:
-            python.version: '3.5'
+          Python37:
+            python.version: '3.7'
       steps:
         - task: UsePythonVersion@0
           inputs:
@@ -227,8 +227,8 @@ stages:
       condition: not(startsWith(variables['Build.SourceBranch'], 'refs/tags'))
       strategy:
         matrix:
-          Python35:
-            python.version: '3.5'
+          Python37:
+            python.version: '3.7'
       steps:
         - task: UsePythonVersion@0
           inputs:
@@ -338,8 +338,8 @@ stages:
         matrix:
           Python36:
             python.version: '3.6'
-          Python37:
-            python.version: '3.7'
+          Python35:
+            python.version: '3.5'
           Python38:
             python.version: '3.8'
       steps:
@@ -393,8 +393,8 @@ stages:
         matrix:
           Python36:
             python.version: '3.6'
-          Python37:
-            python.version: '3.7'
+          Python35:
+            python.version: '3.5'
           Python38:
             python.version: '3.8'
       steps: