Drop lint-py2 and mypy from tox environment

Why: They would become redundant in following commits, which move the
linting workloads over to pre-commit.

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.4.0
 envlist =
-    docs, packaging, mypy, lint, lint-py2,
+    docs, packaging, lint,
     py27, py35, py36, py37, py38, pypy, pypy3
 
 [helpers]
@@ -61,22 +61,3 @@ commands_pre =
 commands =
     flake8
     isort --check-only --diff
-
-[testenv:lint-py2]
-skip_install = True
-basepython = python2
-deps = {[lint]deps}
-commands_pre =
-# No need to flake8 docs, tools & tasks in py2
-commands =
-    flake8 src tests
-    isort --check-only --diff
-
-[testenv:mypy]
-skip_install = True
-basepython = python3
-deps = -r{toxinidir}/tools/requirements/mypy.txt
-commands_pre =
-commands =
-    mypy src
-    mypy src -2