Add pre-commit config

.pre-commit-config.yaml

@@ -0,0 +1,10 @@
+repos:
+- repo: https://github.com/ambv/black
+  rev: 19.10b0
+  hooks:
+  - id: black
+    language_version: python3
+- repo: https://github.com/pre-commit/mirrors-mypy
+  rev: v0.740
+  hooks:
+  - id: mypy

pyproject.toml

@@ -24,13 +24,14 @@ classifiers = [
 ]
 
 [tool.poetry.dependencies]
-python = "^3.7"
+python = "^3.6"
 
 [tool.poetry.dev-dependencies]
 pytest = "^5.2"
 pytest-cov = "^2.7"
 pytest-runner = "^5.1"
-black = { version = "^19.10b0", python = "^3.7" }
+black = { version = "^19.10b0", python = "^3.6" }
+mypy = "0.740"
 pre-commit = "^1.17"
 bw_sphinxtheme = "^0.1.8"