Encrypts connection before passing creds

[00willo]

Ensure appropriate flags are set on ldap3.Connection object based on the following:

• use SSL if ldaps is specified in url config option, ie ldaps on port 636
• explicitly disables ssl, but enables TLS creation before the user bind passes credentials if any tls config is defined. ie ldap on port 389+start_tls

[00willo]

Updated PR to also test with python3.7.

Looks like the Travis build failed on the first commit due to a transient dependency issue, which is now resolved.

[00willo]

@fschulze Do you need anything else added or changed in this PR before it can be merged?

[fschulze]

@00willo time to look at it. I hope to get to it next week. Thanks!

[fschulze]

Work on devpi-ldap was very low priority for a while, so I only got to look at this now.

This PR looks very useful, but the code base has changed a bit in the meantime. Would you still be interested in rebasing and updating this PR? If so, it would be nice to have some tests for the added conditions in the connection setup.