Switch from pkg_resources to importlib.metadata (#2155)

* Switch from pkg_resources to importlib.metadata

importlib.metadata is a standard library replacement for
pkg_resources, available starting with Python 3.8. It is faster than
pkg_resources. This lets the plugin interface use it when available,
i.e. on Python 3.8 and later. On earlier Python, it uses the
importlib_metadata PyPI backport if available, and finally falls back
on pkg_resources. setup.cfg gains an extra called plugins, which can
allow a project to install "pygments[plugins]" in order to ensure that
plugins support is present even if installed with older Python
versions.

Timings for lexing an empty file with a lexer from a random plugin:

Before:

real	0m0,238s
user	0m0,210s
sys	0m0,029s

After:

real	0m0,141s
user	0m0,125s
sys	0m0,017s

Fixes #2116, #2126

1 files changed, 34 insertions, 5 deletions

diff --git a/doc/docs/plugins.rst b/doc/docs/plugins.rst
index 1008013a..67388607 100644
--- a/doc/docs/plugins.rst
+++ b/doc/docs/plugins.rst
@@ -1,6 +1,6 @@
-================
-Register Plugins
-================
+=======
+Plugins
+=======
 
 If you want to extend Pygments without hacking the sources, but want to
 use the lexer/formatter/style/filter lookup functions (`lexers.get_lexer_by_name`
@@ -13,8 +13,37 @@ That means you can use your highlighter modules with the `pygmentize` script,
 which relies on the mentioned functions.
 
 
-Entrypoints
-===========
+Plugin discovery
+================
+
+At runtime, discovering plugins is preferentially done using Python's
+standard library module `importlib.metadata`_, available in Python 3.8
+and higher. In earlier Python versions, Pygments attempts to use the
+`importlib_metadata`_ backport, if available. If not available, a
+fallback is attempted on the older `pkg_resources`_ module. Finally, if
+``pkg_resources`` is not available, no plugins will be loaded at
+all. Note that ``pkg_resources`` is distributed with `setuptools`_, and
+thus available on most Python environments. However, ``pkg_resources``
+is considerably slower than ``importlib.metadata`` or its
+``importlib_metadata`` backport. For this reason, if you run Pygments
+under Python older than 3.8, it is recommended to install
+``importlib-metadata``. Pygments defines a ``plugins`` packaging extra,
+so you can ensure it is installed with best plugin support (i.e., that
+``importlib-metadata`` is also installed in case you are running Python
+earlier than 3.8) by specifying ``pygments[plugins]`` as the
+requirement, for example, with ``pip``:
+
+.. sourcecode:: shell
+
+   $ python -m pip install --user pygments[plugins]
+
+.. _importlib.metadata: https://docs.python.org/3.10/library/importlib.metadata.html
+.. _importlib_metadata: https://pypi.org/project/importlib-metadata
+.. _pkg_resources: https://setuptools.pypa.io/en/latest/pkg_resources.html
+
+
+Defining plugins through entrypoints
+====================================
 
 Here is a list of setuptools entrypoints that Pygments understands: