CLN: remove have_pytz?
From [tslib.pyx](https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslib.pyx#L4083)

```
try:
    import pytz
    UTC = pytz.utc
    have_pytz = True
except:
    have_pytz = False

[...]
def tz_convert_single(int64_t val, object tz1, object tz2):
    [...]
    if not have_pytz:
        import pytz
```

From much [earlier](https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslib.pyx#L63) in tslib.pyx
```
from pytz.tzinfo import BaseTzInfo as _pytz_BaseTzInfo
```

Is the try/except still necessary?  If so, is `import pytz` the right thing to do in `tz_convert_single`?
