[ci] Enclose Python versions in quotes in config yml files (#5015)

* Update .appveyor.yml

* Update .vsts-ci.yml

.appveyor.yml

@@ -3,7 +3,7 @@ version: 3.3.2.99.{build}
 image: Visual Studio 2015
 platform: x64
 configuration:  # a trick to construct a build matrix with multiple Python versions
-  - 3.7
+  - '3.7'
 
 # only build pull requests and
 # commits to 'master'

.vsts-ci.yml

@@ -52,7 +52,7 @@ jobs:
         METHOD: source
         # on Ubuntu 14.04, gpu_source build segfaults with newer version of Python
         # (and newer version of scipy as a consequence)
-        PYTHON_VERSION: 3.7
+        PYTHON_VERSION: '3.7'
       swig:
         TASK: swig
   steps: