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/isa/arm64/__init__.py

@@ -8,6 +8,7 @@ ARMv8 CPUs running the Aarch64 architecture.
 from __future__ import absolute_import
 from . import defs
 from . import settings, registers  # noqa
+from cdsl.isa import TargetISA  # noqa
 
 # Re-export the primary target ISA definition.
-ISA = defs.ISA.finish()
+ISA = defs.ISA.finish()  # type: TargetISA