Remove redundant tests

Since `black` is in your `.pre-commit-config.yaml`, having extra formatting tests is redundant + you would have to manage its version in two more places.
As for running `pre-commit run -a` on the CI this is done by the 'Pre-commit' job, which utilizes caching.

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.4.0
-envlist = py{36,37,38,39},lint,check-formatting,manifest
+envlist = py{36,37,38,39},lint,manifest
 isolated_build = True
 
 [testenv]
@@ -9,18 +9,8 @@ commands = python -m pytest {posargs}
 
 [testenv:lint]
 basepython = python3.8
-deps =
-    pre-commit
-commands = pre-commit run --all-files
-
-; This env just runs `black` and fails tox if it's not formatted correctly.
-; If this env fails on CI, run `tox -e format` locally in order to apply changes.
-[testenv:check-formatting]
-basepython = python3.8
-deps = black==19.10b0
-skip_install = true
-commands =
-    black src tests --diff --check  --config {toxinidir}/pyproject.toml
+deps = pre-commit
+commands = pre-commit run -a
 
 [testenv:docs]
 basepython = python3.8
@@ -35,13 +25,6 @@ deps = check-manifest
 skip_install = true
 commands = check-manifest
 
-[testenv:format]
-basepython = python3.8
-deps = black==19.10b0
-skip_install = true
-commands =
-    black src tests --config {toxinidir}/pyproject.toml
-
 [testenv:generate-badge]
 basepython = python3.8
 commands =
@@ -66,5 +49,5 @@ python =
     3.5: py35
     3.6: py36
     3.7: py37
-    3.8: py38, docs, lint, manifest, check-formatting
+    3.8: py38, docs, manifest
     3.9: py39