upgrade to pymongo v4

[newgene]

We are currently dropping support for python 3.5 and lower (minimal python 3.6+), we should upgrade PyMongo package to its v4.x which also a version requiring python 3.6+:

• Current pymongo dependency:
  

  biothings.api/setup.py

  Line 64 in bb89c4d

  'pymongo>=3.12.0,<4.0', # support MongoDB 5.0 since v3.12.0

• Ref:
  https://pymongo.readthedocs.io/en/stable/migrate-to-pymongo4.html