Support return/raise statements within a version/platform check.

Feature

If a function returns early due to a version check, do not check the remaining code in that function.

Pitch

When writing code for multiple versions of Python, but certain functions/features require specific versions, it can be useful to raise exceptions or return early. Currently, this will only work properly with using an if/else statement. This needlessly indents code and conflicts with PyLint R1705

Example code:

#!/usr/bin/env python3

import sys

# test.py:9: error: Variable annotation syntax is only supported in Python 3.6 and greater
def f() -> None:
        if sys.version_info <= (3,6):
                raise RuntimeError('python 3.6 or greater required')
        x: int = 0
        print(x)


# No mypy error
def g() -> None:
        if sys.version_info <= (3,6):
                raise RuntimeError('python 3.6 or greater required')
        else:
                x: int = 0
                print(x)

Start by running mypy on the two example functions in the issue and compare the diagnostics for the version-check branches. No source file or test is named; done means a return or raise after a supported version/platform check stops checking the remaining function body without requiring an else branch.