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Those changes are also compatible with Sphinx 2.x

The use of .. cpp:enumerator::  for a C enum is questionable,
but .. c:member:: no longer works in Sphinx 3 for the value of
a C enum, but .. c:enumerator:: is Sphinx 3 only
( https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html#directive-c-enumerator )

Sphinx 3 is also more strict with the use of cross-reference
to C types. It rejects cross-references to things like
`` :c:type:`int` `` since this isn't a type we actually define.

Fixes #2028

Equivalent to proj_angular_input() and proj_angular_output() but
checking for degree units instead. proj_create_crs_to_crs() rarely, if
ever, returns pipelines that has radians as input or output so using
proj_angular_*() is not a useful check for io units of pipelines.

These two new functions should make life a bit easier for users that
generally store there angular coordinates in radians.

Closes #2027