Python2: Cannot use dnspython resolver without selectors2

[jamadden]

• gevent version: 20.6
• Python version: Any Py2

Because of a bug in the way the patched import is happening.

$ GEVENT_RESOLVER=dnspython python -c 'import gevent.socket; gevent.socket.gethostbyname("google.com")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "//src/gevent/_socketcommon.py", line 181, in gethostbyname
    return get_hub().resolver.gethostbyname(hostname)
  File "//src/gevent/hub.py", line 830, in _get_resolver
    self._resolver = self.resolver_class(hub=self) # pylint:disable=not-callable
  File "//src/gevent/hub.py", line 826, in resolver_class
    return GEVENT_CONFIG.resolver
  File "//src/gevent/_config.py", line 50, in getter
    return self.settings[setting_name].get()
  File "//src/gevent/_config.py", line 146, in get
    self.value = self.validate(self._default())
  File "//src/gevent/_config.py", line 248, in validate
    return self._import_one_of([self.shortname_map.get(x, x) for x in value])
  File "//src/gevent/_config.py", line 223, in _import_one_of
    return self._import_one(candidates[-1])
  File "//src/gevent/_config.py", line 237, in _import_one
    module = importlib.import_module(module)
  File "/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "//src/gevent/resolver/dnspython.py", line 143, in <module>
    dns = _patch_dns()
  File "//src/gevent/resolver/dnspython.py", line 131, in _patch_dns
    patcher = importer('dns', extra_all, after_import_hook)
  File "//src/gevent/_patcher.py", line 187, in import_patched
    patcher = _SysModulesPatcher(module_name, extra_all)
  File "//src/gevent/_patcher.py", line 55, in __init__
    in iteritems(MAPPING)
  File "//src/gevent/_patcher.py", line 54, in <dictcomp>
    for gevent_name, stdlib_name
  File "//lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "//src/gevent/selectors.py", line 23, in <module>
    import selectors2 as __selectors__
ImportError: No module named selectors2