Update minimum python version to python 3.6.

It will be 5 years since the Python 3.5.0 release (13 Sep 2015), which is the normal EOL period for Python.

.github/workflows/linuxtests.yml

@@ -8,24 +8,24 @@ jobs:
     strategy:
       matrix:
         group: [integrity, integrity2, docs, usage, python, examples, nonode, linkcheck, lint]
-        python: [3.5, 3.8]
+        python: [3.6, 3.8]
         exclude:
           - group: integrity
-            python: 3.5
+            python: 3.6
           - group: integrity2
-            python: 3.5
+            python: 3.6
           - group: docs
-            python: 3.5
+            python: 3.6
           - group: usage
-            python: 3.5
+            python: 3.6
           - group: linkcheck
-            python: 3.5
+            python: 3.6
           - group: nonode
-            python: 3.5
+            python: 3.6
           - group: lint
-            python: 3.5
+            python: 3.6
           - group: examples
-            python: 3.5
+            python: 3.6
       fail-fast: false
     timeout-minutes: 120
     runs-on: ubuntu-latest

setup.py

@@ -18,7 +18,7 @@ from setupbase import (
 from setuptools import setup
 from setuptools.command.develop import develop
 
-min_version = (3, 5)
+min_version = (3, 6)
 
 if sys.version_info < min_version:
     error = """
@@ -146,9 +146,9 @@ setup_args = dict(
         'License :: OSI Approved :: BSD License',
         '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',
     ],
 )
 
@@ -186,7 +186,7 @@ setup_args['extras_require'] = {
 
 setup_args['package_data'] = package_data_spec
 setup_args['include_package_data'] = True
-setup_args['python_requires'] = '>=3.5'
+setup_args['python_requires'] = '>=3.6'
 
 # Force entrypoints with setuptools (needed for Windows, unconditional
 # because of wheels)