mypy: enable strict equality checking

This helps detect bugs in the code.

(It also helps encourage better-structured code,
see I256c080c8 which this spawned.)

Change-Id: Ie7ddb100e4fdfa3dd95fcb1a46704df10e77f7ca
Signed-off-by: Eric Harney <eharney@redhat.com>

setup.cfg

@@ -139,6 +139,7 @@ show_error_codes = true
 pretty = true
 html_report = mypy-report
 no_implicit_optional = true
+strict_equality = true
 
 [options]
 packages = cinder