docs: fix or remove broken links

Author: dhellmann

docs/source/command_ref.rst

@@ -57,7 +57,7 @@ installed.
    * :ref:`scripts-postmkvirtualenv`
    * `requirements file format`_
 
-.. _requirements file format: https://pip.pypa.io/en/latest/reference/pip_install/#requirements-file-format
+.. _requirements file format: https://pip.pypa.io/en/stable/reference/requirements-file-format/
 
 .. _command-mktmpenv:
 

docs/source/developers.rst

@@ -98,7 +98,7 @@ the ``tests`` directory.
 
 .. _shunit2: https://github.com/kward/shunit2
 
-.. _tox: https://tox.testrun.org/
+.. _tox: https://tox.wiki/
 
 .. _developer-templates:
 

docs/source/plugins.rst

@@ -186,8 +186,7 @@ used).
 
 .. seealso::
 
-  * `namespace packages <https://setuptools.readthedocs.io/en/latest/setuptools.html#namespace-packages>`__
-  * `Extensible Applications and Frameworks <https://setuptools.readthedocs.io/en/latest/setuptools.html#extensible-applications-and-frameworks>`__
+  * `namespace packages <https://packaging.python.org/en/latest/guides/packaging-namespace-packages/>`__
 
 The Hook Loader
 ---------------
@@ -257,7 +256,7 @@ messages.
 .. seealso::
 
    * `Standard library documentation for logging <https://docs.python.org/library/logging.html>`__
-   * `PyMOTW for logging <https://www.doughellmann.com/PyMOTW/logging/>`__
+   * `PyMOTW for logging <http://pymotw.com/3/logging/>`__
 
 .. _plugins-extension-points: