python 3.7 compatibility

[francisg-gc]

This pull request fixes #404

I remembered to:

• Update or add unit tests if needed
• Update or add documentation/comments if needed
• Made sure stray files or whitespace didn't get committed
• If significant changes, branch from develop and set to merge into develop
• Read the guidelines for contributing

Changes:

• async is now a reserved word :(
• changed 1 line in wpull/driver/process.py calling asyncio.async to using an internal name _async using getattr

[makew0rld]

When running this with Python 3.8.1, I get this error:

❯ wpull --version                                                                
Traceback (most recent call last):
  File "/home/makeworld/.local/bin/wpull", line 11, in <module>
    load_entry_point('wpull==2.0.3', 'console_scripts', 'wpull')()
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 489, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2852, in load_entry_point
    return ep.load()
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2443, in load
    return self.resolve()
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2449, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/home/makeworld/.local/lib/python3.8/site-packages/wpull/application/main.py", line 4, in <module>
    from wpull.application.builder import Builder
  File "/home/makeworld/.local/lib/python3.8/site-packages/wpull/application/builder.py", line 12, in <module>
    from wpull.application.tasks.download import ProcessTask, ParserSetupTask, ClientSetupTask, ProcessorSetupTask, \
  File "/home/makeworld/.local/lib/python3.8/site-packages/wpull/application/tasks/download.py", line 10, in <module>
    from wpull.processor.coprocessor.phantomjs import PhantomJSParams
  File "/home/makeworld/.local/lib/python3.8/site-packages/wpull/processor/coprocessor/phantomjs.py", line 19, in <module>
    from wpull.driver.phantomjs import PhantomJSDriverParams, PhantomJSDriver
  File "/home/makeworld/.local/lib/python3.8/site-packages/wpull/driver/phantomjs.py", line 17, in <module>
    PhantomJSDriverParams = namedlist.namedtuple(
  File "/home/makeworld/.local/lib/python3.8/site-packages/namedlist.py", line 450, in namedtuple
    type_dict = {'__new__': _make_fn('__new__', _nt_new, fields, defaults),
  File "/home/makeworld/.local/lib/python3.8/site-packages/namedlist.py", line 180, in _make_fn
    code = compile(module_node, '<string>', 'exec')
TypeError: required field "posonlyargs" missing from arguments

Maybe this StackOverflow question can help?

[traverseda]

I believe the right way to do this is self._stderr_reader = asyncio.ensure_future(self._read_stderr())