zodburi: v↑ to 3.0.0 for python3

A new version of zodburi has been released [1] that
removes Python2 compatibility shims [2] and is therefore
needed to successfully run 'wendelin.core' tests with python3 [3].

Suggested by @kirr.

[1] https://pypi.org/project/zodburi/3.0.0/
[2] https://github.com/Pylons/zodburi/commit/a00fea755
[3] wendelin.core!35

/reviewed-by @kirr @jerome
/reviewed-on !1745