Pin mpmath to v1.3.0.

v1.4.0 introduced a change to mpc_div that changed mpmath's calculation of sinc(inf) from nan to inf.

See change: mpmath/mpmath@93c0647.

PiperOrigin-RevId: 874083009

Author: danielsuo

build/test-requirements.txt

@@ -3,7 +3,7 @@ cloudpickle
 filelock==3.20.3
 flatbuffers
 hypothesis==6.142.1  # TODO(justinfu): Fix test failures surfaced by Hypothesis 6.147.0 and remove pin.
-mpmath>=1.3
+mpmath==1.3.0  # TODO(dsuo): Remove pin sinc(inf) is resolved as either nan or 0.0.
 pillow>=11.3
 portpicker
 pytest<9.0  # Works around https://github.com/pytest-dev/pytest/issues/13895