Bump lint jobs to latest stable python version

The python version used for pylint can effect results returned, pylint
is very sensitive to the environment it's run in. Running on python 3.5
which is the oldest supported python version masks issues on newer
python versions. For example, if you have python 3.7 installed locally
lint will always fail becaues of changes to the stdlib. This starts to
become more of an issue as a particular python version ages, since the
user and contributor base moves to newer versions of python. This commit
changes the version of pylint we run in CI to 3.7 to tests the leading
edge instead of the trailing so we keep our pylint testing up to date
with the python versions we support.

Related to Qiskit#3127 and Qiskit#1179

azure-pipelines.yml

@@ -161,8 +161,8 @@ 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: