removed support for python older than python 3.6

Author: GabrieleCalarota

requirements-dev.txt

@@ -3,4 +3,4 @@ mock
 pytest
 pytest-cov
 tox
-python-telegram-bot==5.0
+python-telegram-bot

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py30,py34,py35,py36,coverage
+envlist = py36,py37,py38,py39,coverage
 
 [testenv]
 skip_install = True
@@ -9,7 +9,7 @@ deps=
 commands=py.test tests --cov-fail-under 90 --color=auto --cov=telegram_handler --cov-report=term-missing
 
 [testenv:coverage]
-basepython = python3.5
+basepython = python3.6
 passenv = CI TRAVIS_BUILD_ID TRAVIS TRAVIS_BRANCH TRAVIS_JOB_NUMBER TRAVIS_PULL_REQUEST TRAVIS_JOB_ID TRAVIS_REPO_SLUG TRAVIS_COMMIT
 deps =
     codecov>=1.4.0