drop unsupported versions of python

python 3.5 hasn't been updated since 2014 and left official support in 2020
https://endoflife.date/python

Author: jacalata

.travis.yml

@@ -2,10 +2,11 @@ language: python
 cache: pip
 
 python:
-  - "2.7"
-  - "3.4"
-  - "3.5"
-  - "3.6"
+  - "3.6" # EOL 23 Dec 2021
+  - "3.7"
+  - "3.8"
+  - "3.9"
+  - "3.10"
   - "pypy"
 # command to install dependencies
 install: