Lean on pkgutil.resolve_name for importing apps

[kgaughan]

With #445, it no longer makes sense not to use pkgutil.resolve_name to handle the importing of WSGI applications in the runner. There is one very minor behavioural change in that pkgutil.resolve_name allows a broader syntax for the imports, but supports the same syntax as the existing code being removed, so this is a minor concern.

It has the pleasant side effect of removing some legacy code only present for Python 2.6 that was missed when Python 2 support was removed.

[kgaughan]

Very peculiar. Some of the test workflows randomly hung and had to be rerun. It looks like they failed just as test_function.py was being run. After a few attempts, I was able to reproduce similar behaviour locally:

platform linux -- Python 3.12.7, pytest-8.3.3, pluggy-1.5.0
rootdir: .../projects/waitress
configfile: setup.cfg
testpaths: tests
plugins: cov-5.0.0
collected 796 items                                                                                                                                                                                                                                                                                                                                                                         

tests/test_adjustments.py .................................................                                                                                                                                                                                                                                                                                                           [  6%]
tests/test_buffers.py ....................................................                                                                                                                                                                                                                                                                                                            [ 12%]
tests/test_channel.py .........................................................................................................................                                                                                                                                                                                                                                       [ 27%]
tests/test_functional.py ...................................................................................^CTraceback (most recent call last):
  File ".../projects/waitress/src/waitress/server.py", line 325, in run
    self.asyncore.loop(
  File ".../projects/waitress/src/waitress/wasyncore.py", line 245, in loop
    poll_fun(timeout, map)
  File ".../projects/waitress/src/waitress/wasyncore.py", line 183, in poll
    read(obj)
  File ".../projects/waitress/src/waitress/wasyncore.py", line 104, in read
    obj.handle_read_event()
  File ".../projects/waitress/src/waitress/wasyncore.py", line 466, in handle_read_event
    self.handle_read()
  File ".../projects/waitress/src/waitress/channel.py", line 156, in handle_read
    data = self.recv(self.adj.recv_bytes)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../projects/waitress/src/waitress/wasyncore.py", line 409, in recv
    def recv(self, buffer_size):
    
  File ".../projects/waitress/.venv/lib/python3.12/site-packages/coverage/collector.py", line 252, in lock_data
    self.data_lock.acquire()
  File ".../projects/waitress/tests/test_functional.py", line 43, in sigterm
    sys.exit(0)
SystemExit: 0

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.12/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/usr/lib/python3.12/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File ".../projects/waitress/tests/test_functional.py", line 33, in start_server
    svr(app, queue, **kwargs).run()
  File ".../projects/waitress/src/waitress/server.py", line 331, in run
    self.task_dispatcher.shutdown()
  File ".../projects/waitress/src/waitress/task.py", line 118, in shutdown
    def shutdown(self, cancel_pending=True, timeout=5):
    
  File ".../projects/waitress/.venv/lib/python3.12/site-packages/coverage/collector.py", line 252, in lock_data
    self.data_lock.acquire()
KeyboardInterrupt

At first glance, coverage looks like the culprit, but it might just be an innocent bystander. At the very least, there looks to be a race condition somewhere.

[kgaughan]

I opened #449 to deal with the issue functional tests hanging.