TypeError when executing user_sync.py via python3

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## Issue
Executing the LDAP [`user_sync.py`](https://github.com/linkedin/oncall/blob/master/src/oncall/bin/user_sync.py) script via `python@3.6.9` results in the following error:
`TypeError: a bytes-like object is required, not 'str'` (full output shown [here](https://gist.github.com/dk0r/9d17ae5e4531ee54ec2f986a6c3b846d))

## Expected Behavior
As described [here](https://oncall.tools/docs/admin_guide.html#adding-users), it is expected that the [`user_sync.py`](https://github.com/linkedin/oncall/blob/master/src/oncall/user_sync/ldap_sync.py) script should periodically sync users from a preexisting LDAP server

## Steps to reproduce
Migrate [`oncall@1.2.3`](https://github.com/linkedin/oncall/tree/v1.2.3) to python3.
Step-by-step instructions for getting oncall running via py3 are published [here](https://gist.github.com/dk0r/844c8284aea8dee6a3fe9f44ff91e875
)

## Troubleshooting attempts:
* I'm not a python dev but google indicates the above error may be due to an upstream issue w/ `gevent` running on py3.
Apparently something in `gevent` may need to be encoded to a bytes object via `.encode('utf-8')` 🤷🏻‍♂️
* Posted issue to irisoncall's [gitter](https://gitter.im/irisoncall/Lobby?at=5f441ba9ddc2d041c0c4d757)

@diegocepedaw @dwang159

Start with src/oncall/bin/user_sync.py and src/oncall/user_sync/ldap_sync.py, then reproduce the failure under Python 3.6.9 using the linked migration instructions and traceback. The issue is done when the LDAP user sync runs under Python 3 without the reported TypeError.

Written by the indexing model from the issue text.