*: 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:

wendelin.core@d97641d2

and in Python 2.7.15:

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
/reviewed-on !550

software/neoppod/software-common.cfg

@@ -118,7 +118,7 @@ url = ${:_profile_base_location_}/my.cnf.in
 md5sum = 9f6f8f2b5f4cb0d97d50ffc1d3837e2f
 
 [versions]
-BTrees = 4.4.1
+BTrees = 4.5.1
 ZODB = 4.4.5
 coverage = 4.5.1
 ecdsa = 0.13