Skip building trees for nodes with invalid tag name

[oleksandr-pavlyk]

This change attempts to exclude arguments with invalid tag names from being inserted into the TreeBuilder as per comment from gh-5552.

It does resolve the exception reported in gh-5552 and in gh-5686, but I am not sure whether this is the best way to implement the idea to skip nodes with invalid names.

Please feel free to suggest improvements.

[oleksandr-pavlyk]

@da-woods @scoder Please look this change over and provide guidance.

[da-woods]

The main thing it's missing is a test - probably just to check that a generator expression doesn't break it the build

def f(a, b):
    return (x for x in range(a, b))

is probably sufficient to hit the bug I think.

Unfortunately I'm not really sure how gdb mode is tested: probably in Cython/Debugger/Tests but I've never looked in there myself

[oleksandr-pavlyk]

I attempted to validate the fix by locally running cygdb, but could not get it to work.

python ../cython/cython.py --cplus --gdb a.pyx --output-file a.cxx
g++ -fPIC a.cxx $(python3-config --includes) $(python3-config --ldflags) -shared -o a$(python3-config --extension-suffix)

Then

(dev_dpctl) opavlyk@opavlyk-mobl:~/repos/cython-crash$ cygdb
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word".
There was an error in Python code originating from the file /home/opavlyk/miniconda3/envs/dev_dpctl/lib/python3.9/site-packages/Cython/Debugger/Cygdb.py
It used the Python interpreter /usr/bin/python
Traceback (most recent call last):
  File "<string>", line 11, in <module>
ModuleNotFoundError: No module named 'Cython'

[tornaria]

Works for me. It fixes the minimal example I made up for #5552, but in fact it fixes compilation of the whole of sagemath.

A test as suggested above (or as the one made up for #5552) would be a good thing. Bear in mind that the failure only occurs when lxml python package is installed.

[scoder]

I'm not sure that we really need to change start() and end() as well, but it's probably better to be consistent.

[scoder]

Looks good to me now.

[scoder]

Thanks