Stop testing Python 3.5 as it gives inconsistent results.

For some reason Python 3.5 (on GitHub's actions only) inconsistently
fails it's tests. It's also the only version that compiles gevent and
is long-EOL, and official support will be dropped alongside 2.7 in v2.

Author: Fizzadar

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04]
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
+        python-version: [2.7, 3.6, 3.7, 3.8]
         include:
           - os: macos-10.15
             python-version: 3.8