*: v↑ BTrees to 4.5.1

BTrees 4.4.1 is from early 2017. BTrees 4.5.0 contains fix for crash when deallocating items:

https://github.com/zopefoundation/BTrees/issues/75
https://github.com/zopefoundation/BTrees/commit/16e70dd337

It is similar crash and fix to this one in wendelin.core:

https://lab.nexedi.com/nexedi/wendelin.core/commit/d97641d2ba

and in Python 2.7.15:

https://lab.nexedi.com/nexedi/slapos/commit/8e098385ab#note_72575

BTrees 4.5.1 is a small maintenance release over 4.5.0 and is currently latest BTree release.

/cc @jm, @vpelletier, @kazuhiko, @jerome