Rename "GCC_DIAGNOSTIC" macro to make it Cython specific and make it available to other utility code sections.

Mainly by moving it to a separate section to make it easier
to find, however also added a small amount of extra information
about some of the obvious limitations.

* Cython debugger documentation: Added link to an installation script.
* Got a new libpython.py from the cpython source distribution.
* Default language level in tests is now 3 instead of 2
* Migrated codefile from python 2 to python 3.
* Added testcase for the cy list command in cygdb.
* Temporarily removing test case that freezes gdb.
* Fixed a bug that broke several Cygdb tests.

The cython_debug/cython_debug_info_* files map the names of the C-functions generated by the Cython compiler to the names of the functions in the *.pyx source. If the function was defined using "def" (and not "cpdef" or "cdef") in the *.pyx source file, the C-function named in cython_debug/cython_debug_info_* used to be __pyx_pw_*, which is the name of the wrapper function and now it is __pyx_f_*, which is the name of the actual function. This makes some Cygdb tests pass that did not pass before.

* Better error messages: If a cygdb command raises, a traceback will be printed.
* Fixed a bug in cygdb.

The following now works:
1. Start cygdb
2. Type "cy exec" and hit enter
3. Type some other lines
4. Type "end" and hit enter.
-> These "other lines" will get executed

* Fixed a bug in cygdb: cy list now works outside of functions.
* Added print_hr_allmarkers function for easier debugging.
* Fixed a bug that broke cygdb:

cy break did not work if you put the breakpoint outside of a function if there was e.g. the following somewhere in your *.pyx file:
cdef class SomeClass():
    pass

* Added a Cygdb test for printing global variables.
* Fixing cygdb: Replaced cy print with a simple, working solution.
* If an exception in Cygdb occurs, a stacktrace will be printed.
* Fixed a bug that broke cy break -p
* Bugfix: The compiler now writes out correctly which cython linenumber and path corresponds to which c linenumber.
* Set language_level=2 in runtests.py

Fixes https://github.com/cython/cython/issues/1071

There's now a fairly wide range of valid syntax for declaring things in pyx files and it should all be supported when cimporting from them.

This reverts commit a333d6a8.

The change did not resolve the test issue in travis.

Closes https://github.com/cython/cython/issues/3775

This is a follow up to https://github.com/cython/cython/pull/3307

Closes https://github.com/cython/cython/pull/3763

Use NumPy 1.18.x for testing on travis as long as 1.19.[01+] ships a numpy.pxd that is incompatible with Cython 3.0.

Try to fix NumPy test failures by not setting the "NPY_NO_DEPRECATED_API" #define for NumPy 1.19.[01].

Change "Py_intptr_t" declaration in numpy.pxd to what CPython uses as fallback. "int" seems overly lazy if it tends to be larger on many systems.