Bump pytz version to >= 2018.9

[ivan-transferwise]

Hello ✋

Is there any good reason why you have this constraint pytz==2018.4? It makes it incompatible with ZenPy, which requires at least 2018.9. ZenPy library is also used in tap-zendesk (which is now using very old buggy version of ZenPy==2.0.0).

So, can we bump the version of pytz?

Thanks in advance.

Came here to create this issue. Can someone please look into this?