DEP: Fix escaped string characters deprecated in Python 3.6.

In Python 3.6 a number of escape sequences that were previously accepted
-- for instance "\(" that was translated to "\\(" -- are deprecated. To
retain the previous behavior either raw strings must be used or the
backslash must be properly escaped itself.

Old value was erroneously obtained on a sorted array which is a best
case for the median of 3 pivoted introsort.

The apply_along_axis path is significantly more expensive than currently
accounted for in the check. Increase the minimum axis size from 400 to
1000 elements.
Either apply_along_axis got more expensive over time or the original
benchmarking was flawed.

This requires to base masked median on sort(endwith=False) as we need to
distinguish Inf and NaN.
Using Inf as filler element of the sort does not work as then the mask is not
guaranteed to be at the end.
Closes gh-8340

Also fixed 1d ma.median not handling np.inf correctly, the nd variant
was ok.

Closes gh-8220

DOC:  See also np.load and np.memmap in np.lib.format.open_memmap

BUG: ediff1d should return subclasses