stubgen: use sys.version_info, rather than hardcoding 3.6 (#10907)

Not too familiar with stubgen, so maybe there's a good reason for
hardcoding, e.g. to ensure stubs capture sys.version checks in the
source? Most stubs in typeshed use sys.version checks for a) checking
Python 2, b) if they're stdlib backports. So hopefully this is a
reasonable change.

Fixes #10905

Author: hauntsaninja

mypy/stubgen.py

@@ -1661,7 +1661,7 @@ def parse_options(args: List[str]) -> Options:
 
     ns = parser.parse_args(args)
 
-    pyversion = defaults.PYTHON2_VERSION if ns.py2 else defaults.PYTHON3_VERSION
+    pyversion = defaults.PYTHON2_VERSION if ns.py2 else sys.version_info[:2]
     if not ns.interpreter:
         ns.interpreter = sys.executable if pyversion[0] == 3 else default_py2_interpreter()
     if ns.modules + ns.packages and ns.files: