ModuleNotFoundError: No module named 'gevent' with pyscopg2

Since version 3.0b2, I am experiencing a warning with following code

import ZODB.config

conf = """
%import relstorage
<zodb main>
    <relstorage>
        keep-history false
        <postgresql>
            dsn host=localhost dbname=rspg
        </postgresql>
    </relstorage>
</zodb>
"""
db = ZODB.config.databaseFromString(conf)

Despite the warning, the connection is open and can be used.

The warning is:

Unable to import driver
Traceback (most recent call last):
  File "[...]/site-packages/relstorage/adapters/drivers.py", line 135, in __init__
    self.driver_module = mod = self.get_driver_module()
  File "[...]/site-packages/relstorage/adapters/postgresql/drivers/psycopg2.py", line 173, in get_driver_module
    __import__('gevent')
ModuleNotFoundError: No module named 'gevent'
Unable to import driver
Traceback (most recent call last):
  File "[...]/site-packages/relstorage/adapters/drivers.py", line 135, in __init__
    self.driver_module = mod = self.get_driver_module()
  File "[...]/site-packages/relstorage/adapters/postgresql/drivers/psycopg2.py", line 173, in get_driver_module
    __import__('gevent')
ModuleNotFoundError: No module named 'gevent'

I have tested this against several combinations of python3 versions and relstorage versions, and psycopg2 2.8.4 (each test in a separate virtualenv, detroyed and created before running test script; and each test repeated 5 times):

python 3.6 + relstorage 3.0b1 : no warning
python 3.7 + relstorage 3.0b1 : no warning
python 3.8 + relstorage 3.0b1 : no warning
python 3.6 + relstorage 3.0b2 : 4 warning on 5 tests
python 3.7 + relstorage 3.0b2 : 3 warning on 5 tests
python 3.8 + relstorage 3.0b2 : 4 warning on 5 tests
python 3.6 + relstorage 3.0b3 : 2 warning on 5 tests
python 3.7 + relstorage 3.0b3 : 3 warning on 5 tests
python 3.8 + relstorage 3.0b3 : 3 warning on 5 tests

I am quite puzzled with randomness of warnings; but the warning occurred frequently enough to report an issue.

zodb / relstorage

ModuleNotFoundError: No module named 'gevent' with pyscopg2 #375