Additional matplotlib 3.1 deprecation warnings

[bwohlberg]

The 0.7.1 release has addresses some of the MatplotlibDeprecationWarning warnings with matplotlib 3.1, but there are some additional ones remaining:

/opt/conda/miniconda3/envs/py36/lib/python3.6/site-packages/mpldatacursor/datacursor.py:413: MatplotlibDeprecationWarning:
The pprint_val function was deprecated in Matplotlib 3.1 and will be removed in 3.3.
return formatter.pprint_val(x)
/opt/conda/miniconda3/envs/py36/lib/python3.6/site-packages/mpldatacursor/pick_info.py:297: MatplotlibDeprecationWarning:
The line2d_seg_dist function was deprecated in Matplotlib 3.1 and will be removed in 3.3.
i, (p0, p1) in enumerate(edges)]

[jondo]

According to #100 (comment), at least the pprint_val warning has been fixed on master.