Use astropy pixel_to_pixel instead of internal calc_pixmap function

[jdavies-st]

Use astropy.wcs.utils.pixel_to_pixel instead of

drizzle/drizzle/calc_pixmap.py

Line 5 in 291b8b6

def calc_pixmap(first_wcs, second_wcs):

if possible.

This will require bumping up the astropy and other dependency versions.

[mcara]

Not sure what this is referring to but, with a soon to be released 2.0, this issue is no longer valid: the caller can use any method to create a pixel map.

[jdavies-st]

This issue is still valid. The calc_pixmap() function maintained here in Drizzle is a specialized version of the more general function in astropy

https://github.com/astropy/astropy/blob/92cf1d46dc165fc3e07fba373a7211b814560fbb/astropy/wcs/utils.py#L887

The functionality has moved upstream to Astropy and therefore Drizzle should use it.