src/sage/libs/mpmath/ext_main.pyx: handle ugly mpmath output
There's a test in this file that is trying to use the global property
mp.pretty=True but is failing on my machine. Since the test is inside
a TESTS block and not visible to end users, the simplest way to fix it
is to expect the ugly output from mpmath; although in one instance we
do call str() on an mpmath real number to obtain '2.3' instead of some
2.999... thing that would require a tolerance.
