Update testing: add docs + lint jobs; use pre-commit for linting (#426)

* Move docs and lint to their own [3.8] build job for more parallelism

* No codecov for docs or lint

* Move isort into pre-commit

* Add some handy linters to pre-commit

* Add rst-backticks linter and fix the errors

* Add pyupgrade and add upgrades

* Test docs and lint on GitHub Actions

* Xenial is default

.pre-commit-config.yaml

@@ -0,0 +1,25 @@
+repos:
+  - repo: https://github.com/asottile/pyupgrade
+    rev: v1.25.1
+    hooks:
+      - id: pyupgrade
+        args: ["--py3-plus"]
+
+  - repo: https://github.com/pre-commit/mirrors-isort
+    rev: v4.3.21
+    hooks:
+      - id: isort
+        additional_dependencies: [toml]
+
+  - repo: https://github.com/pre-commit/pygrep-hooks
+    rev: v1.4.2
+    hooks:
+      - id: python-check-blanket-noqa
+      - id: rst-backticks
+
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v2.4.0
+    hooks:
+      - id: check-merge-conflict
+      - id: check-toml
+      - id: check-yaml