mypy: Enable stricter configuration

We're quite close to being able to enable only "strict = True" but not
quite there. In the mean time enable some useful individual settings.

disallow_untyped_defs:
    report an error whenever code contains a function definition
    without type annotations
warn_redundant_casts:
    report an error whenever code uses an unnecessary cast
warn_unused_ignores:
    report an error whenever code uses an unnecessary # type: ignore
    comment
warn_unreachable:
    report an error whenever code is determined to be unreachable or
    redundant

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

setup.cfg

@@ -9,6 +9,11 @@ ignore =
 
 [mypy]
 warn_unused_configs = True
+warn_redundant_casts = True
+warn_unused_ignores = True
+warn_unreachable = True
+strict_equality = True
+disallow_untyped_defs = True
 files = tuf/api/
 
 [mypy-securesystemslib.*]