drop the final remaining use of the deprecated pkg_resources module

Declaring a namespace package has gone through a few revisions.
pkg_resources has a version that is heavily deprecated. pkgutil provides
a python2/python3 compatible version that is also compatible with native
python3 namespaces.

https://packaging.python.org/en/latest/guides/packaging-namespace-packages/

pkg_resources is very very deprecated and importing or using it results
in deprecation warnings. It's time to move off of it entirely.

Fixes: PyFilesystem#577

fs/__init__.py

@@ -1,7 +1,7 @@
 """Python filesystem abstraction layer.
 """
 
-__import__("pkg_resources").declare_namespace(__name__)  # type: ignore
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)
 
 from . import path
 from ._fscompat import fsdecode, fsencode

fs/opener/__init__.py

@@ -3,7 +3,7 @@
 """
 
 # Declare fs.opener as a namespace package
-__import__("pkg_resources").declare_namespace(__name__)  # type: ignore
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)
 
 # Import opener modules so that `registry.install` if called on each opener
 from . import appfs, ftpfs, memoryfs, osfs, tarfs, tempfs, zipfs

setup.cfg

@@ -43,7 +43,7 @@ setup_requires =
     setuptools >=38.3.0
 install_requires =
     appdirs~=1.4.3
-    setuptools
+    setuptools          ; python_version < '3.8'
     six ~=1.10
     enum34 ~=1.1.6      ;  python_version < '3.4'
     typing ~=3.6        ;  python_version < '3.6'