add mypy tox environment and dependencies

setup.py

@@ -119,6 +119,15 @@ setup(
             'flake8-docstrings',
             'pep8-naming',
         ],
+        'mypy': [
+            'mypy',
+            'types-Pillow',
+            'types-urllib3',
+            'types-beautifulsoup4',
+            'types-PyYAML',
+            'types-requests',
+            'types-Flask-Cors',
+        ],
 
         # Plugin (optional) dependencies:
         'absubmit': ['requests'],

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py38-{cov,lint}, docs
+envlist = py38-{cov,lint,mypy}, docs
 
 [_test]
 deps = .[test]
@@ -13,15 +13,23 @@ deps = .[test]
 deps = .[lint]
 files = beets beetsplug beet test setup.py docs
 
+[_mypy]
+deps =
+    .[mypy]
+    .[test]
+
 [testenv]
 deps =
     {test,cov}: {[_test]deps}
     lint: {[_lint]deps}
+    mypy: {[_mypy]deps}
 passenv = INTEGRATION_TEST
 commands =
     test: python -bb -m pytest -rs {posargs}
     cov: coverage run -m pytest -rs {posargs}
     lint: python -m flake8 {posargs} {[_lint]files}
+    mypy: mypy -p beets -p beetsplug
+    mypy: mypy test
 
 [testenv:docs]
 basepython = python3.10