Fixes for mypy 0.600 (#324)

* Remove the mypy version constraint and set strict_optional to False.

* Add type annotations for `ISA` variables.

mypy 0.600 seems to require explicit annotations here.

* Annotate the ISA variables in the defs.py files too.

lib/codegen/meta/mypy.ini

@@ -2,3 +2,4 @@
 disallow_untyped_defs = True
 warn_unused_ignores = True
 warn_return_any = True
+strict_optional = False