Fix Python version logic that would fail with Py 4 (Netflix#792)

Fix error discovered by https://github.com/asottile/flake8-2020

% `flake8 . --count --select=E9,F63,F7,F82,Y --show-source --statistics`
```
./metaflow/metaflow/util.py:53:36: YTT204 `sys.version_info.minor` compared to integer (python4), compare `sys.version_info` to tuple
if sys.version_info.major >= 3 and sys.version_info.minor >= 7:
                                   ^
```

metaflow/util.py

@@ -50,7 +50,7 @@ def unquote_bytes(x):
 
     from shlex import quote as _quote
 
-if sys.version_info.major >= 3 and sys.version_info.minor >= 7:
+if sys.version_info >= (3, 7):
     from collections import namedtuple
 
     namedtuple_with_defaults = namedtuple