Move minimum Python version to 3.6

Python 3.5 has been EOL since Sept 2020, 10 months ago at this point.

The latest release of PyGithub does not support 3.5 (#1770), thus remove
the support here also.

Also add test cases for Python 3.9, which is the current stable release.

.travis.yml

@@ -17,12 +17,12 @@ install: pip install -U tox
 language: python
 matrix:
   include:
+  - env: TOXENV=py39
+    python: 3.9.1
   - env: TOXENV=py38
     python: 3.8.2
   - env: TOXENV=py37
     python: 3.7.7
   - env: TOXENV=py36
     python: 3.6.10
-  - env: TOXENV=py35
-    python: 3.5.9
 script: tox

setup.py

@@ -54,13 +54,13 @@
         'Natural Language :: English',
         "Programming Language :: Python",
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: Implementation :: CPython',
     ],
-    python_requires=">=3.5",
+    python_requires=">=3.6",
     entry_points={
         'console_scripts': [
             'pyup = pyup.cli:main',

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35, py36, py37, py38
+envlist = py36, py37, py38, py39
 
 [testenv]
 setenv =