Relaxes requirements for pytz

[Lucas-Mc]

This change relaxes the requirements for the pytz package from >=2018.3 to >= 2017.2 since it is only required by pandas which only requires pytz>=2017.2.

[tompollard]

Could you explain why it is necessary to relax the requirement? What is the pytz package and which version are we running on the server?

[Lucas-Mc]

From the most recent PhysioNet build:

The conflict is caused by:
    The user requested pytz==2017.2
    django 2.2.13 depends on pytz
    google-api-core 1.13.0 depends on pytz
    pandas 1.1.0 depends on pytz>=2017.2
    wfdb 3.1.2 depends on pytz>=2018.3

That original requirement was too strict since when I removed it from requirements.txt for WFDB-Python only >=2017.2 was required. The version should be 2017.2 based on the requirements.txt for PhysioNet.