More pypy3 fixes

Fixes a segfault on pypy3 due to the coroutine type missing tp_iternext in 5.8.

Last change accidentally disallowed mismatching 'nogil' assignments even when compatible. Reallow them, and clean up the last change a little.

Allow assignment of C function pointers with compatible exception declarations (not only exact matches).

Allow mismatches of broader exception declarations in .pxd signatures as long the implemented signature is covered.

Pass pyx_library kwargs to py_library

Previously, using "is_f_contig" and "is_c_contig" together would leave one of them undeclared.
Closes #1872.

Make vtable order of extension types with fused methods only dependant on the original declaration order (e.g. in the .pxd file).
Previously, fused methods were specialised and expanded on first use, which lead to an arbitrary order in the vtable.
Also fixes compile failures when inheriting from base types with fused cdef methods.
Fixes #1873.

Ensure extra arguments, like visibility, can still be applied to pyx_libraries.

PyPy 3 fixes

Note that on pypy, SOABI tags start with '.pypy' which contains
the string '.py'...

It is used even when CYTHON_USE_TYPE_SLOTS is disabled.

Disable it on pypy3, since it doesn't support PEP 442 yet.

Remove special case support for block name suffix after "XYZ.proto" utility code title. Replaced by existing "@proto_block" option.

Refactor coercion predicate function to make it clearer what happens for the template based conversions and for the non-template string conversions.

Avoid deprecation warning for "Py_UNICODE_strlen()" by always using the version that we ship anyway.