Update README on pyximport

Closes #1859.

Closes #1860.

Release buffer on validation errors. My guess is that we previously just leaked the buffer owner in that case.

Closes #1858.

Check result of "declare_builtin()" because it might a) not have been found and b) not actually be a builtin. In both cases, it's actually not a literal.

Add missing special signature for "__getattribute__" method in order to enforce it and mark the method as special.

Replace closure class special casing from slot generation code and replace it with saner and more general "is_special" check.

Repair slot signature typing for "__richcmp__" special method after adding synthesis from "__eq__" etc. methods.
Add test for "__richcmp__" override of existing "__eq__" etc. methods.

Generate "tp_richcompare" slot when "__eq__" and/or its friends are defined but "__richcmp__" is not.
Closes #690.

Implement @cython.exceptval() decorator to make the "except x" signature declaration available in pure Python mode.
Closes #1653.

Need to delay annotation typing after the "as_cfunction()" conversion in AdjustDefByDirectives transform as we do not have an 'env' yet.

Work around compiler crash due to missing 'env' (before declaration analysis) when combining @ccall with argument annotations.

Allow arbitrary type references in annotations, rather than just valid executable Python expressions.

Disable an "embedsignatures" test that is not actually valid with PEP 484 but fails to compile as the return type now interpreted as a C-tuple of Python objects (which is not supported).

Change annotation typing directive to cover all type annotations and modify annotation parsing to be more PEP 484 compatible.
Only "cython.*" types are interpreted as C types from now on, everything else is considered Python types for maximum compatibility with Python type hints.