setup: Introduce typecheck alias.

setup.cfg

@@ -1,2 +1,7 @@
 [aliases]
 test=pytest
+typecheck=pytest --addopts="--mypy conservancy_beancount"
+
+[mypy]
+show_error_codes = True
+warn_unused_configs = True

setup.py

@@ -13,8 +13,14 @@ setup(
     install_requires=[
         'beancount>=2.2',
     ],
-    setup_requires=['pytest-runner'],
+    setup_requires=[
-    tests_require=['pytest'],
+        'pytest-mypy',
+        'pytest-runner',
+    ],
+    tests_require=[
+        'mypy>=0.770',
+        'pytest',
+    ],
 
     packages=['conservancy_beancount'],
     entry_points={},