Mypy does not catch possibly undefined variables

[Dakkaron]

Mypy does not catch variables that might not be defined due to ifs and therelike:

if False:
	a = "Test"

print(a)

This happens both in Python2- and Python3-mode
I remember in an earlier version I always had to define them before the if, so that the variables are always defined:

a = ""

if condition:
	a = "Test"

print(a)

[rwbarton]

Duplicate of #2400.