Drop Python 3.6 support

pyproject.toml

@@ -9,7 +9,7 @@ readme = "README.md"
 authors = [
     { name = "Masashi Shibata", "email" = "mshibata@preferred.jp" }
 ]
-requires-python = ">=3.6"
+requires-python = ">=3.7"
 license = {text = "MIT License"}
 classifiers = [
     "Development Status :: 3 - Alpha",
@@ -17,10 +17,10 @@ classifiers = [
     "License :: OSI Approved :: MIT License",
     "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",
     "Programming Language :: Python :: 3 :: Only",
     "Intended Audience :: Science/Research",
 ]

tox.ini

@@ -1,6 +1,5 @@
 [tox]
 envlist =
-    py36
     py37
     py38
     py39
@@ -9,10 +8,6 @@ envlist =
     isort
     mypy
 
-[testenv:py36]
-basepython = python3.6
-commands = python -m unittest {posargs}
-
 [testenv:py37]
 basepython = python3.7
 commands = python -m unittest {posargs}