chore: add mypy to the build (#689)

The method used here follows the same pattern that SQLAlchemy has been using to add types. For an example, see https://github.com/sqlalchemy/sqlalchemy/pull/9039/files and note the removal of the `# mypy: ignore-errors` comment to incrementally add types.

4 files changed, 136 insertions, 3 deletions

diff --git a/.github/workflows/testing.yml b/.github/workflows/testing.yml
index e65486d..fd60e86 100644
--- a/.github/workflows/testing.yml
+++ b/.github/workflows/testing.yml
@@ -41,7 +41,7 @@ jobs:
       - name: Code check
         run: tox -e ${TOX_VENV}
         env:
-          TOX_VENV: black,pep8
+          TOX_VENV: black,pep8,mypy
 
   test:
     needs: [validate]
diff --git a/pyproject.toml b/pyproject.toml
index 0ec882e..b7d67f7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -21,3 +21,125 @@ extend-exclude = '''
 
 [tool.pytest.ini_options]
 addopts = "-ra -v"
+
+[tool.mypy]
+
+# For details on each flag, please see the mypy documentation at:
+# https://mypy.readthedocs.io/en/stable/config_file.html#config-file
+
+# Note: The order of flags listed here should match the order used in mypy's
+# documentation to make it easier to find the documentation for each flag.
+
+# Import Discovery
+ignore_missing_imports = false
+
+# Disallow dynamic typing
+disallow_any_unimported = true
+disallow_any_expr = false
+disallow_any_decorated = true
+disallow_any_explicit = true
+disallow_any_generics = true
+disallow_subclassing_any = true
+
+# Untyped definitions and calls
+disallow_untyped_calls = true
+disallow_untyped_defs = true
+disallow_incomplete_defs = true
+check_untyped_defs = true
+disallow_untyped_decorators = true
+
+# None and Optional handling
+implicit_optional = false
+strict_optional = true
+
+# Configuring warnings
+warn_redundant_casts = true
+warn_unused_ignores = true
+warn_no_return = true
+warn_return_any = true
+warn_unreachable = true
+
+# Miscellaneous strictness flags
+allow_untyped_globals = false
+allow_redefinition = false
+local_partial_types = true
+implicit_reexport = false
+strict_concatenate = true
+strict_equality = true
+strict = true
+
+# Configuring error messages
+show_error_context = true
+show_column_numbers = true
+hide_error_codes = false
+pretty = true
+color_output = true
+error_summary = true
+show_absolute_path = true
+
+# Miscellaneous
+warn_unused_configs = true
+verbosity = 0
+
+# FIXME: As type annotations are introduced, please remove the appropriate
+#        ignore_errors flag below. New modules should NOT be added here!
+
+[[tool.mypy.overrides]]
+module = [
+    'kazoo.client',
+    'kazoo.exceptions',
+    'kazoo.handlers.eventlet',
+    'kazoo.handlers.gevent',
+    'kazoo.handlers.threading',
+    'kazoo.handlers.utils',
+    'kazoo.hosts',
+    'kazoo.interfaces',
+    'kazoo.loggingsupport',
+    'kazoo.protocol.connection',
+    'kazoo.protocol.paths',
+    'kazoo.protocol.serialization',
+    'kazoo.protocol.states',
+    'kazoo.recipe.barrier',
+    'kazoo.recipe.cache',
+    'kazoo.recipe.counter',
+    'kazoo.recipe.election',
+    'kazoo.recipe.lease',
+    'kazoo.recipe.lock',
+    'kazoo.recipe.partitioner',
+    'kazoo.recipe.party',
+    'kazoo.recipe.queue',
+    'kazoo.recipe.watchers',
+    'kazoo.retry',
+    'kazoo.security',
+    'kazoo.testing.common',
+    'kazoo.testing.harness',
+    'kazoo.tests.conftest',
+    'kazoo.tests.test__connection',
+    'kazoo.tests.test_barrier',
+    'kazoo.tests.test_build',
+    'kazoo.tests.test_cache',
+    'kazoo.tests.test_client',
+    'kazoo.tests.test_counter',
+    'kazoo.tests.test_election',
+    'kazoo.tests.test_eventlet_handler',
+    'kazoo.tests.test_exceptions',
+    'kazoo.tests.test_gevent_handler',
+    'kazoo.tests.test_hosts',
+    'kazoo.tests.test_interrupt',
+    'kazoo.tests.test_lease',
+    'kazoo.tests.test_lock',
+    'kazoo.tests.test_partitioner',
+    'kazoo.tests.test_party',
+    'kazoo.tests.test_paths',
+    'kazoo.tests.test_queue',
+    'kazoo.tests.test_retry',
+    'kazoo.tests.test_sasl',
+    'kazoo.tests.test_security',
+    'kazoo.tests.test_selectors_select',
+    'kazoo.tests.test_threading_handler',
+    'kazoo.tests.test_utils',
+    'kazoo.tests.test_watchers',
+    'kazoo.tests.util',
+    'kazoo.version'
+]
+ignore_errors = true
diff --git a/setup.cfg b/setup.cfg
index 562e3a7..027c6d1 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -73,11 +73,14 @@ sasl =
 docs =
     Sphinx>=1.2.2
 
+typing =
+    mypy>=0.991
+
 alldeps =
     %(dev)s
     %(eventlet)s
     %(gevent)s
     %(sasl)s
     %(docs)s
-
+    %(typing)s
 
diff --git a/tox.ini b/tox.ini
index 5ecc403..6fd246f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,7 +4,7 @@ requires=
     virtualenv>=20.7.2
 skip_missing_interpreters=True
 envlist =
-    pep8,black,
+    pep8,black,mypy,
     gevent,eventlet,sasl,
     docs,
     pypy3
@@ -59,3 +59,11 @@ deps =
 usedevelop = True
 commands = black --check {posargs: {toxinidir}/kazoo {toxinidir}/kazoo}
 
+[testenv:mypy]
+basepython = python3
+extras = alldeps
+deps =
+    mypy
+    mypy: types-mock
+usedevelop = True
+commands = mypy --config-file {toxinidir}/pyproject.toml {toxinidir}/kazoo