DOC: Fix misleading description of random_sample in numpy.random docstring.
MAINT: double underscores in Lapack-lite C module

Strictly speaking, any non-standard name with "__" in it causes
undefined behavior in C. It's also completely unnecessary to use
any in this module.
MAINT: C identifiers causing UB + useless char returns

C identifiers aren't allowed to match "^_[A-Z]". Fixed that in
numpyos.c and removed some more useless underscores.

Changed NumPyOS_ascii_tolower and the code that calls it to handle
ints, not chars. C promotes char to int in so many situations that
char variables are just a pain. (The C standard library uses int
in <ctype.h>, for example.)
MAINT: unused variable, formatting
MAINT: typo
MAINT: fix item_selection.c so it compiles without warnings
Merge pull request #3950 from larsmans/c-fixes

C fixes
ENH: unroll vector minmax loop

Improves speed by 10% on intel cpus.
Simplify code by moving the fenv support check into the dispatcher. fenv works
on all common platforms (including windows), the fallback is not worth it for
the exotic platforms where it might not be available.
ENH: accept callable as message in assert_

Allows deferring evaluation until failure.
Used for blocked minmax test which evaluate array representations for
the error message many thousand times accumulating to a full second
useless runtime.
ENH: High time consumption by PyUFunc_GetPyValues in ufunc_object.c
For every single operation calls, numpy has to extract value of buffersize, errormask
 and name to pack and build error object. These two functions, _extract_pyvals and
PyUFunc_GetPyValues together use >12% of time.
Merge pull request #3945 from juliantaylor/minmax-improve

ENH: unroll vector minmax loop
MAINT: refactor ufunc error object handling

_get_global_ext_obj: retrieves global ufunc object
_get_bufsize_errmask: get only bufsize and errormask from ufunc object
_extract_pyvals: handle NULL extobj

PyUFunc_GetPyValues implemented as _get_global_ext_obj +_extract_pyvals
drop unused first_error variable.

fix errobj memory leak in previous commit.

add some test for the extobj and warning path, the warning tests are
disabled like the raising path as they fail on a bunch of platforms.
Merge pull request #3946 from juliantaylor/pyufunc_refactor

delay error object creation to when errors occur
ENH: Add scalar special cases to Priority getter

Exact Python scalars can never have a priority, but checking
it is expensive. This adds checks for these to the Priority
getter function.
Merge pull request #3951 from seberg/scalar-priority

ENH: Add scalar special cases to Priority getter
Merge pull request #3921 from mdickinson/doc-numpy-random-fixes

DOC: Fix misleading description of random_sample in numpy.random docstring.
BUG: core: ensure __r*__ has precedence over __numpy_ufunc__

Add a special case to the implementation of ndarray.__mul__ et al. that
refuses to work on other objects that are not ndarray subclasses and
implement both __numpy_ufunc__ and __r*__.

This way, execution passes first to the custom __r*__ method, which
makes it possible to have e.g. __mul__ and np.multiply do different
things.

Additionally, disable one __array_priority__ special case handling when
also __numpy_ufunc__ is defined.
Merge pull request #3856 from pv/op-before-ufunc

BUG: core: ensure __r*__ has precedence over __numpy_ufunc__
MAINT: io: handle a bad fmt argument in savetxt by raising a ValueError.
BUG: #2052 del scalar subscript causes segfault

Fixes issue #2052, where attempting to delete a scalar field causes a
segfault. Returns ValueError instead, like when attempting to delete an
array element.

Also added a test for this bug.