Dropped testing for Python 3.5

The resulting XML files for Python 3.5 are different between consecutive
runs, while the version of Yosys and version of lxml (and other
packages) are the same.

Signed-off-by: Grzegorz Latosinski <glatosinski@antmicro.com>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 452ce71..b44ffeb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,8 +2,6 @@
 
 matrix:
   include:
-    - python: 3.5
-      env: TOXENV=py35
     - python: 3.6
       env: TOXENV=py36
     - python: 3.7