Fix mpmath version to 1.3.0 due to incompatibility with sympy (quantumlib#6477)

Fixes quantumlib#6475

Author: kenya-sk

cirq-core/requirements.txt

@@ -2,6 +2,9 @@
 
 duet>=0.2.8
 matplotlib~=3.0
+# Temporary fix for https://github.com/sympy/sympy/issues/26273
+# TODO: remove once `pip install --pre sympy` works in a fresh environment
+mpmath<=1.3.*
 networkx>=2.4
 numpy~=1.16
 pandas