Hopefully a very ungrateful bug has been removed. When you used to invoke some request through update handlers, it could potentially enter an infinite loop. This has been mitigated and it's now safe to invoke things again! A lot of updates were being dropped (all those gzipped), and this has been fixed too.
More bug fixes include a correct parsing of certain TLObjects thanks to @stek29, and some wrong calls that would cause the library to crash thanks to @andr-04, and the ReadThread not re-starting if you were already authorized.
Internally, the .to_bytes() function has been replaced with __bytes__ so now you can do bytes(tlobject).