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/riscv/defs.py

@@ -7,7 +7,7 @@ from __future__ import absolute_import
 from cdsl.isa import TargetISA, CPUMode
 import base.instructions
 
-ISA = TargetISA('riscv', [base.instructions.GROUP])
+ISA = TargetISA('riscv', [base.instructions.GROUP])  # type: TargetISA
 
 # CPU modes for 32-bit and 64-bit operation.
 RV32 = CPUMode('RV32', ISA)