Fix hard-coded pytype version (#4771)

[tests/pytype_test.py](./tests/pytype_test.py#L176) is still using
Python `3.6`:
> python_version="2.7" if version == 2 else "3.6",

The test fails to run if `python3.6` is unavailable (for example on
Debian 10 Buster, which has 3.7).

According to (https://devguide.python.org/#status-of-python-branches
`3.6` is still maintained until end of 2021.

`pytype` itself [claims](https://github.com/google/pytype#requirements)
to run on `2.7`, `3.5`-`3.8`.

Change the script to use the version of Python from the invoking
interpreter.

tests/pytype_test.py

@@ -13,6 +13,7 @@ will also discover incorrect usage of imported modules.
 import argparse
 import os
 import re
+import sys
 import traceback
 from typing import List, Match, Optional, Sequence, Tuple
 
@@ -173,7 +174,7 @@ def run_all_tests(*, files_to_test: Sequence[Tuple[str, int]], typeshed_location
         stderr = (
             run_pytype(
                 filename=f,
-                python_version="2.7" if version == 2 else "3.6",
+                python_version="2.7" if version == 2 else "{0.major}.{0.minor}".format(sys.version_info),
                 typeshed_location=typeshed_location,
             )
             if not dry_run