Remove usage of deprecated module pkg_resources

[matrss]

Pyfilesystem2 makes use of this module which got deprecated at some point. This leads to noisy DeprecationWarning's in the test suite of a project using pyfilesystem2.

To reproduce:

>>> import warnings
>>> warnings.resetwarnings()
>>> import fs
/home/icg149/mambaforge/envs/fs-report/lib/python3.11/site-packages/fs/__init__.py:4: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
  __import__("pkg_resources").declare_namespace(__name__)  # type: ignore
/home/icg149/mambaforge/envs/fs-report/lib/python3.11/site-packages/fs/__init__.py:4: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('fs')`.
Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
  __import__("pkg_resources").declare_namespace(__name__)  # type: ignore
/home/icg149/mambaforge/envs/fs-report/lib/python3.11/site-packages/fs/opener/__init__.py:6: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('fs.opener')`.
Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
  __import__("pkg_resources").declare_namespace(__name__)  # type: ignore
/home/icg149/mambaforge/envs/fs-report/lib/python3.11/site-packages/pkg_resources/__init__.py:2350: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('fs')`.
Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
  declare_namespace(parent)

[matrss]

I just now saw that there is PR #553 for this.