Linchpin clean installation with Python 3 requires pymongo

[abraverm]

Describe the bug
linchpin up fails on missing pymongo module:

Traceback (most recent call last):
  File "/opt/linchpin3/.venv/bin/linchpin", line 11, in <module>
    load_entry_point('linchpin', 'console_scripts', 'linchpin')()
  File "/opt/linchpin3/.venv/lib64/python3.7/site-packages/pkg_resources/__init__.py", line 489, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/opt/linchpin3/.venv/lib64/python3.7/site-packages/pkg_resources/__init__.py", line 2793, in load_entry_point
    return ep.load()
  File "/opt/linchpin3/.venv/lib64/python3.7/site-packages/pkg_resources/__init__.py", line 2411, in load
    return self.resolve()
  File "/opt/linchpin3/.venv/lib64/python3.7/site-packages/pkg_resources/__init__.py", line 2417, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/opt/linchpin3/linchpin/__init__.py", line 23, in <module>
    from linchpin.rundb.drivers import DB_DRIVERS
  File "/opt/linchpin3/linchpin/rundb/drivers.py", line 4, in <module>
    from .mongodb import MongoDB
  File "/opt/linchpin3/linchpin/rundb/mongodb.py", line 10, in <module>
    from pymongo import MongoClient, DESCENDING
ModuleNotFoundError: No module named 'pymongo'

To Reproduce
Steps to reproduce the behavior:

1. Create python 3 virtual env python3 -m venv .venv
2. Install linchpin from repo: pip install -e .
3. Run linchpin up

Expected behavior
If mongo driver is not used then it shouldn't be imported