Dependabot is doing weird stuff updating certifi, let's DIY (#943)

#893
and
#895

are trying to update certifi to fix a security vulnerability, but it's
running into problems with protobuf and seems to be downgrading
google-cloud-datastore no good reason.

I temporarily added certifi to Pipfile and ran "pipenv update certifi"
to generate these Pipfile.lock changes

Author: andrewpollock