unpin z3 version

4.12.2.0 does not work out of the box on python 3.12 because of a missing transitive dependency on setuptools, no longer included by default. Later z3 versions address that problem.

(int-blast and other goodies are also available)

Fixes a16z#273

requirements.txt

@@ -1 +1 @@
-z3-solver==4.12.2.0
+z3-solver>=4.13