fix: build error caused by removed py2neo package

On Oct. 10, py2neo package was abruptly removed from pypi, GitHub, and
the py2neo website now displays just a super funny meme: https://py2neo.org/

Yes, we should get rid of that dependency, but we are still supposed to
support existing users. So we install py2neo from our fork.

changelog.d/20231010_111112_regis_fix_py2neo.md

@@ -0,0 +1 @@
+- [Bugfix] Fix build error caused by py2neo package that was abruptly pulled from pypi and GitHub. (by @regisb)