ldap3.ServerPool usage appears to be hardcoded

It's impossible to use this library without the ldap3.ServerPool functionality because it appears to be hardcoded into here.

I get `raise LDAPServerPoolError('pools can be exhausted only when checking for active servers')` but that's after I set `LDAP_AUTH_POOL_ACTIVE = False`.

I was definitely not expecting it to completely crash when changing a boolean default value like this. Is this an oversight or am I missing something obvious?

I'm using version 0.15.8