[CI/CD] Fix Tox SSL error in Windows Travis job

The tox sdist-make step failed with SSL: CERTIFICATE_VERIFY_FAILED so
fix by install certifi to ensure updated SSL certificates are available.

.travis.yml

@@ -43,9 +43,10 @@ jobs:
       os: windows
       language: shell
       before_install:
+        # Python version must match available deluge-libtorrent
         - choco install python --version 3.6.8
         - python --version
-        - python -m pip install --upgrade pip
+        - python -m pip install --upgrade pip certifi
         - python -m pip install deluge-libtorrent
       env:
         - PATH=/c/Python36:/c/Python36/Scripts:$PATH