py-ldap3: updated to 2.7

2.7:
- tested against Python 3.8.1 and pyasn1 0.4.8
- re-enabled ssl exception raising on bad certificate when only 1 server is present in the server pool
- removed Python 2.6 from Travis configuration
- added support for source specifications in LDAP connections
- added support for allowing special AD security identifier (SID) in DN
- fixed pickling of entry and attribute
- close connection when auto_bind fails
- operational attributes can be used in Abstraction Layer
- additional SSL options can be used in Tls object
- threading.Event replaces loop checking in async strategy. ASYNC strategy should be much faster now
- adding a key that is already an alias that contains other aliases in CaseInsensitiveWithAliasDict() now works properly
- when searching for GUID, UUID and SID the backslash character (0x5C) is properly managed
- LDIF output properly formatted when controls are missing
- operational attributes are not returned in MOCK strategies when not requested
- undecodable values are returned as raw bytes when using the pyasn1 decoder

(adam)