Minor cleanup regarding __Pyx_PyMethod_New

Make sure to include "Python.h" also from the "public" header file, which depends on its declarations.
Closes #3133.

Remove duplicated line from __Pyx_c_pow

When compiling to C++ code and using automatic conversion from Python
str to C++ string, Cython inserts this function template but generates the warning:
warning: string.from_py:15:63: local variable 'length' might be referenced before assignment

* Special-case setting __new__ in class

* Always bind Cython functions

* TEST: add cgetter test for pointer and pxd use
* BUG: handle more AttributeNode-with-property-decorator
* BUG: fix numpy/__init__.pxd
* ENH: add @property for ndarray.size, formatting cleanup

Add an assertion for the assumption that all identifier start characters are also valid continuation characters, so that we notice it if that ever changes.

Clean up the Lexicon.py generation script and use f-strings to prevent accidentally running it with older Python versions.

Closes #2601