MAINT: Help boost::python libraries at least not crash

This adds an almost random "sanity check" to `PyArray_EquivTypes`
for the sole purpose of allowing boost::python compiled libs to
_not crash_.
boost::python is buggy, it needs to be fixed.  This may break them
(I am unsure), because some conversions which used to work may fail
here (if they worked, they only worked because random type data may
have matched up correctly for our scalar types).

We could error, or warn or... but I hope boost::python will just fix
this soon enough and future us can just delete the whole branch.

Replaces gh-20507

BUG: Fix issues (mainly) found using pytest-leaks (#20583)

MAINT: Fix two minor typing-related problems 

BUG: Fix leaks found using pytest-leaks

MAINT: Check for buffer interface support rather than try/except

BUG: Fix PyInit__umath_linalg type

Use PyMODINIT_FUNC instead of PyObject *.

This checks for buffer interface support using `PyObject_CheckBuffer`
up-front.  That seems to shave off about 100ns e.g. when importing
pandas DataFrames (which do not support the buffer interface, but
do support `__array__`.
The reason is that building a TypeError and then clearing it takes
time.

It felt like a clear, easy win, since the check should be so fast
that checking twice (in the case that it is indeed a buffer) does
not really matter.

But, if nobody likes to think about it just close :).