[MATH] Fix MATH normalization

[lewtun]

Closes #140 by using the official _fix_fracs() function from the MATH repo.

Colab to test out the fix: https://colab.research.google.com/drive/1WumhmGpAoKbA-YaWwDB4gSSgKBZHF_i1?usp=sharing

Here's the code snippet that was triggering the error and now produces the correct output:

s = "The fraction $\\frac{1}{2y+1}$ fails to be defined only if the denominator is zero. This occurs when $y$ is the solution of the equation $$2y+1=0,$$ which is $y=-\\frac 12$. Thus the domain of $k(y)$ is $$\\boxed{\\left(-\\infty,-\\frac 12\\right)\\cup \\left(-\\frac 12,\\infty\\right)}.$$"
math_normalizer(s, is_gold=True)
 # (-\infty,-\frac{1}{2})\cup(-\frac{1}{2},\infty) 

TODO

• Run over some public models to validate scores look OK.

[clefourrier]

LGTM! (and thanks for the docstring)
If tests pass, feel free to merge