[python-experimental] Removes python2 (#6991)

* Removes future from python-exp v3 sample

* Removes future from python-exp v2 sample

* Deletes future from remaining python-exp files

* Removes six from python-exp templates

* Removes six from python-exp samples

* Removes mock from python-exp

* Python-exp switched to py3

* Removes python 2.7 for python-exp ci testing

* Requires python>=3.3 for python-exp

* Reverts unnecessary changes to two templates

samples/openapi3/client/petstore/python-experimental/test_python.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+
+REQUIREMENTS_FILE=dev-requirements.txt
+REQUIREMENTS_OUT=dev-requirements.txt.log
+SETUP_OUT=*.egg-info
+VENV=venv
+DEACTIVE=false
+
+export LC_ALL=en_US.UTF-8
+export LANG=en_US.UTF-8
+
+### set virtualenv
+if [ -z "$VIRTUAL_ENV" ]; then
+		virtualenv $VENV --no-site-packages --always-copy
+		source $VENV/bin/activate
+    DEACTIVE=true
+fi
+
+### install dependencies
+pip install -r $REQUIREMENTS_FILE | tee -a $REQUIREMENTS_OUT
+
+### run tests
+tox || exit 1
+
+### static analysis of code
+#flake8 --show-source petstore_api/
+
+### deactivate virtualenv
+#if [ $DEACTIVE == true ]; then
+#    deactivate
+#fi