Pop on unreadable files

[konraddysput]

If the file doesn't exist, we're popping from the source code collection and this action generates an error, because we're in the middle of iteration.

Error details:

Exception in thread Thread-1 (_worker):
Traceback (most recent call last):
  File "/usr/local/lib/python3.13/threading.py", line 1043, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/usr/local/lib/python3.13/threading.py", line 994, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/backtracepython/report_queue.py", line 32, in _worker
    self.process(report, attachments)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/backtracepython/report_queue.py", line 42, in process
    self.source_code_handler.collect(report)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/backtracepython/source_code_handler.py", line 45, in collect
    for source_code_path in source_code:
                            ^^^^^^^^^^^
RuntimeError: dictionary changed size during iteration

ref: INS-985

[melekr]

only comment i had was around invalid char for python2 which is adressed.

If tests are green, LGTM!