Removal of deprecated :mod:`matplotlib.pylab` code

Lots of code inside the :mod:`matplotlib.mlab` module which was deprecated

in Matplotlib 2.2 has been removed. This means the following functions are

no longer available in the `matplotlib.pylab` module:

- ``amap``

- ``base_repr``

- ``binary_repr``

- ``bivariate_normal``

- ``center_matrix``

- ``csv2rec`` (use `numpy.recarray.tofile` instead)

- ``dist`` (use `numpy.hypot` instead)

- ``dist_point_to_segment``

- ``distances_along_curve``

- ``entropy`` (use `scipy.stats.entropy` instead)

- ``exp_safe``

- ``fftsurr``

- ``find`` (use ``np.nonzero(np.ravel(condition))`` instead)

- ``frange`` (use `numpy.arange` instead)

- ``get_sparse_matrix``

- ``get_xyz_where``

- ``griddata``

- ``identity`` (use `numpy.identity` instead)

- ``inside_poly``

- ``is_closed_polygon``

- ``ispower2``

- ``isvector``

- ``l1norm``

- ``l2norm``

- ``log2`` (use `numpy.log2` instead)

- ``longest_contiguous_ones``

- ``longest_ones``

- ``movavg``

- ``norm_flat`` (use ``numpy.linalg.norm(a.flat, ord=2)`` instead)

- ``normpdf`` (use `scipy.stats.norm.pdf` instead)

- ``path_length``

- ``poly_below``

- ``poly_between``

- ``prctile`` (use `numpy.percentile` instead)

- ``prctile_rank``

- ``rec2csv`` (use `numpy.recarray.tofile` instead)

- ``rec_append_fields``

- ``rec_drop_fields``

- ``rec_join``

- ``rk4`` (use `scipy.integrate.ode` instead)

- ``rms_flat``

- ``segments_intersect``

- ``slopes``

- ``stineman_interp``

- ``vector_lengths``