support alias for `sys` when checking version_info

Issue kind:

Bug Report.

Env:

• mypy 0.620
• Python 3.7.2

How to reproduce

Code as following:

# t.py
import sys as py_sys

if py_sys.version_info < (3, 0):
    unicode = unicode
    chr = unichr
else:
    unicode = str

exec mypy t.py, and use default config, it reports

t.py:5: error: Cannot determine type of 'unicode'
t.py:6: error: Name 'unichr' is not defined

But following code is ok.

# t.py
import sys

if sys.version_info < (3, 0):
    unicode = unicode
    chr = unichr
else:
    unicode = str

Master branch(mypy-0.660+dev.d1c8f27f696012085ca904e96f8b93b92b74919e mypy-extensions-0.4.1 typed-ast-1.1.1) has the same problem.

Except:

mypy can recognize sys module alias, and handle compact code.

Actual:

If use a sys module's alias to get Python version, and write some compact code, it will check all code.

Start with the t.py reproduction and run mypy using the direct sys import, then the py_sys alias, to compare version_info handling and the reported unicode and unichr errors. Done means mypy recognizes the aliased sys module and checks the compact version-dependent code consistently with the direct import.