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/defs.py

@@ -8,7 +8,7 @@ from cdsl.isa import TargetISA, CPUMode
 import base.instructions
 from base.legalize import narrow
 
-ISA = TargetISA('arm64', [base.instructions.GROUP])
+ISA = TargetISA('arm64', [base.instructions.GROUP])  # type: TargetISA
 A64 = CPUMode('A64', ISA)
 
 # TODO: Refine these