Covert tol to float before passing to ipopt.

[moorepants]

Fixes #235.

[moorepants]

Getting this on some mac builds:

=========================== short test summary info ============================
FAILED cyipopt/tests/unit/test_scipy_ipopt_from_scipy.py::TestSLSQP::test_minimize_unbounded_approximated - AssertionError: b'Algorithm stopped at a point that was converged, not to "desired" tolerances, but to "acceptable" tolerances (see the acceptable-... options).'
======= 1 failed, 96 passed, 7 skipped, 1 xpassed, 43 warnings in 11.40s =======
Error: Process completed with exit code 1.