Add typing dependencies for requests, dateutil, setuptools and yaml.

Without these, mypy complains that "Library stubs are not installed".

setup.cfg

@@ -44,6 +44,16 @@ install_requires =
 packages = find:
 python_requires = >=3.6
 
+[options.extras_require]
+test =
+  mypy>=0.770
+  pytest>=3.0
+  pytest-mypy
+  types-requests
+  types-python-dateutil
+  types-setuptools
+  types-PyYAML
+
 [options.entry_points]
 console_scripts =
   accrual-report = conservancy_beancount.reports.accrual:entry_point
@@ -75,6 +85,10 @@ deps =
   mypy>=0.770
   pytest>=3.0
   pytest-mypy
+  types-requests
+  types-python-dateutil
+  types-setuptools
+  types-PyYAML
 
 # Beancount includes type declarations but not the `py.typed` flag file mypy
 # is looking for to know that. Create it ourselves.