Incorrect tzinfo check in tslibs.parsing
https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/parsing.pyx#L339

```            
    if isinstance(tzdata, datetime.tzinfo):
                tzinfo = tzdata
```

If this were ever hit it would raise an error because `datetime.tzinfo` refers to the tzinfo attached to a datetime object, _not_ the tzinfo class defined in the datetime module.

There's basically no path by which this would be reached because we never pass a `tzdata` to this function.  So two things should happen: fix this to check for the correct object, and either remove unreachable code or make it reachable.