Drop Python 3.6 support

The interpreter goes EoL in ~1 month, and it is not supported by
`sphinx-theme-builder`.

Author: pradyunsg

.github/workflows/tests.yml

@@ -59,7 +59,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
         include:
           - python-version: 3.9
             dev: true

setup.py

@@ -5,7 +5,6 @@
 
 from setuptools import setup
 
-
 # from https://packaging.python.org/guides/single-sourcing-package-version/
 HERE = os.path.abspath(os.path.dirname(__file__))
 
@@ -54,7 +53,7 @@ def find_version(*file_paths):
         "coverage": ["pytest-cov", "codecov", *tests_require],
         "dev": ["pyyaml", "pre-commit", "nox"],
     },
-    python_requires=">=3.5",
+    python_requires=">=3.7",
     classifiers=[
         "Development Status :: 4 - Beta",
         "Programming Language :: Python :: 3",