Drop end-of-life Python versions

As per the release cycle (https://devguide.python.org/versions/):

* Python 3.6 EOL since end of 2021
* Python 3.7 EOL since end of 2022

Dropping support for those in future releases.

.github/workflows/ci.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: [3.6, 3.7, 3.8, 3.9, '3.10']
+        python: [3.8, 3.9, '3.10']
 
     steps:
       - uses: actions/checkout@v2

setup.py

@@ -23,11 +23,8 @@
     long_description_content_type="text/markdown",
     classifiers=[
         "Topic :: Utilities",
-        "Programming Language :: Python :: 3.4",
         "Programming Language :: Python",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.6",
-        "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3.10",

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py3{6,7,8,9,10},black,pylama
+envlist = py3{8,9,10},black,pylama
 skip_missing_interpreters = true
 
 [testenv]