Move from pkg_resources to importlib.resources

[DMRobertson]

The documentation for pkg_resources declares the package to be deprecated:

Use of pkg_resources is discouraged in favor of importlib.resources, importlib.metadata, and their backports (importlib_resources, importlib_metadata). Please consider using those libraries instead of pkg_resources.

It would be nice (though by no means critically important) to follow this advice. The standalone backport moduleimportlib_resources provides a migration guide.

Grepping the source tree for pkg_resources, we currently use:

• pkg_resources.resource_filename
• pkg_resources.resource_stream
• pkg_resources.parse_version

The first two are covered by the migration guide. The third can be replaced by packaging.version.Version. We already depend on packaging >= 16.1, and AFAICS from its changelog the Version type and its functionality we use have been present since packaging's initial release.

Once this and #12478 is done, we can remove the development dependency on types-setuptools.

Task list:

• Migrate uses of resource_filename to importlib.resources
• Migrate uses of resource_stream to importlib.resources
• Migrate uses of parse_version to packaging.version.Version
• Remove dev dependency on setuptools and update poetry lockfile (poetry lock --no-update)
• Run linter script
• Run tests (in CI for a PR)

[DMRobertson]

Having tried this out (many thanks @vheuken), it wasn't an easy 1-1 replacement.

The sticking point is that we pass template directories to Jinja2, falling back to a default template directory distributed as a package resource.

synapse/synapse/config/_base.py

Lines 273 to 282 in 4586119

	# Append the default directory at the end of the list so Jinja can fallback on it
	# if a template is missing from any custom directory.
	search_directories.append(self.default_template_dir)
	# TODO: switch to synapse.util.templates.build_jinja_env
	loader = jinja2.FileSystemLoader(search_directories)
	env = jinja2.Environment(
	loader=loader,
	autoescape=jinja2.select_autoescape(),
	)

But importlib.resources makes it tricky to get hold of that path. (It's concerned with providing access to specific file-like resources, which may not correspond to a filesystem directory).

#12542 (comment) has more details. I think it's best to drop this for now.