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:

	nexedi/wendelin.core@d97641d2

and in Python 2.7.15:

	nexedi/slapos@8e098385 (comment 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