revert: "ci: install Python 3.6 that is required by newer version of html5validator to fix the build on TravisCI"

This reverts commit 0e09fac.

During migration to GitHub Actions (#1154), in commit 661444c we
have started to use containers with Ubuntu 20.04 that has Python 3.8.10 so this workaround isn't
needed anymore.

Relate to #1533

Author: php-coder

.travis.yml

@@ -20,11 +20,7 @@ before_script:
       mysql -u root -e 'CREATE DATABASE mystamps CHARACTER SET utf8;';
       mkdir -p /tmp/uploads /tmp/preview;
       cp src/main/resources/test/test.png /tmp/uploads/1.png;
-      sudo add-apt-repository -y ppa:jblgf0/python;
-      sudo apt-get -qq update;
-      sudo apt-get install -y python3.6 python3-pip;
-      curl https://bootstrap.pypa.io/pip/3.6/get-pip.py | sudo -H python3.6;
-      pip3 install --user ansible==2.9.2;
+      pip install --user ansible==2.9.2;
       if [ "$TRAVIS_PULL_REQUEST" != 'false' ]; then
         gem install danger:8.2.2 nokogiri:1.10.10 --no-document;
       fi;