tools: Migrate & merge mypy config into setup.cfg.

Configuration for mypy was previously loading from mypy.ini, however
there was also unconnected - unused - configuration in setup.cfg.

This commit mainly migrates config across from mypy.ini, as well as
setting a python_version (3.6) and ignoring imports with no typing
information. This was broadly the configuration (as unused) previously
in setup.cfg, with the unknown import ignore now applied to all modules
rather than relying upon explicit listing (based on an approach in
zulip/zulip for mypy.ini).

Migrating the mypy configuration into setup.cfg reduces the number of
config files at the base level and reduces the potential confusion over
multiple configuration locations which were previously present.

One marginal benefit is also that fewer root level files pushes the
start of the README further up slightly on GitHub, improving its
visibility.

setup.cfg

@@ -47,9 +47,22 @@ exclude=.git,__pycache__,build,dist,tools
 
 [mypy]
 python_version = 3.6
-mypy_path = mypy_stubs
 
-[mypy-urwid,zulip,pudb]
+# Logistics of what code to check and how to handle the data.
+# mypy_path = stubs/
+scripts_are_modules = True
+show_traceback = True
+cache_dir = .mypy_cache
+
+# Options to make the checking stricter.
+check_untyped_defs = True
+disallow_untyped_defs = True
+disallow_any_generics = True
+warn_no_return = True
+no_implicit_optional = True
+strict_optional = True
+
+# If a library is typed, that's fine, otherwise don't worry
 ignore_missing_imports = True
 
 [coverage:run]