Make backend dependencies optional

docs/install.rst

@@ -9,3 +9,17 @@ To install :mod:`audbackend` run:
     $ # virtualenv --python=python3 ${HOME}/.envs/audbackend
     $ # source ${HOME}/.envs/audbackend/bin/activate
     $ pip install audbackend
+
+Per default,
+only the file-system backend will be installed.
+To install all backends run:
+
+.. code-block:: bash
+
+    $ pip install audbackend[all]
+
+or select single backends, e.g.
+
+.. code-block:: bash
+
+    $ pip install audbackend[artifactory]

pyproject.toml

@@ -28,13 +28,21 @@ classifiers = [
 ]
 dependencies = [
     'audeer >=1.20.0',
-    'dohq-artifactory >=0.8.1',
     'pywin32; sys_platform == "win32"'
 ]
 # Get version dynamically from git
 # (needs setuptools_scm tools config below)
 dynamic = ['version']
 
+[project.optional-dependencies]
+artifactory = [
+    'dohq-artifactory >=0.8.1',
+]
+all = [
+    'dohq-artifactory >=0.8.1',
+]
+
+
 [project.urls]
 repository = 'https://github.com/audeering/audbackend/'
 documentation = 'https://audeering.github.io/audbackend/'