docs: fix mkdocs_plugin installation

This patch fixes the error caused by deprecated setup.py in
mkdocs_plugin. The setup.py is rewritten into pyproject.toml according
to the docs:
https://packaging.python.org/en/latest/specifications/pyproject-toml/#pyproject-toml-spec

Signed-off-by: Milosz Wasilewski <milosz.wasilewski@oss.qualcomm.com>
diff --git a/.github/workflows/docs.yml b/.github/workflows/docs.yml
index ec7551c..fc6bd92 100644
--- a/.github/workflows/docs.yml
+++ b/.github/workflows/docs.yml
@@ -21,7 +21,7 @@
 
       - name: Install current plugin
         run: |
-         python mkdocs_plugin/setup.py install --force
+         pip install mkdocs_plugin/
          pip freeze
 
       - name: Build docs
diff --git a/mkdocs_plugin/pyproject.toml b/mkdocs_plugin/pyproject.toml
new file mode 100644
index 0000000..bc8a4c7
--- /dev/null
+++ b/mkdocs_plugin/pyproject.toml
@@ -0,0 +1,30 @@
+[build-system]
+requires = [
+	"setuptools",
+	"mkdocs>=1.1",
+	"tags-macros-plugin@git+https://github.com/mwasilew/mkdocs-plugin-tags.git"
+]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "mkdocs-test-definitions-plugin"
+version = "1.5"
+keywords = [
+	"mkdocs",
+	"python",
+	"markdown",
+	"wiki"
+]
+requires-python = ">=3.5"
+license = "GPL-2.0-or-later"
+
+authors = [
+	{name = "Milosz Wasilewski", email = "milosz.wasilewski@oss.qualcomm.com"}
+]
+
+[project.urls]
+Repository = "https://github.com/linaro/test-definitions"
+GitHub = "https://github.com/linaro/test-definitions"
+
+[project.entry-points."mkdocs.plugins"]
+linaro-test-definitions = "testdefinitionsmkdocs:LinaroTestDefinitionsMkDocsPlugin"
diff --git a/mkdocs_plugin/setup.py b/mkdocs_plugin/setup.py
deleted file mode 100644
index 6820658..0000000
--- a/mkdocs_plugin/setup.py
+++ /dev/null
@@ -1,33 +0,0 @@
-from setuptools import setup, find_packages
-
-
-setup(
-    name="mkdocs-test-definitions-plugin",
-    version="1.5",
-    description="An MkDocs plugin that converts LAVA test definitions to documentation",
-    long_description="",
-    keywords="mkdocs python markdown wiki",
-    url="https://github.com/linaro/test-definitions",
-    author="Milosz Wasilewski",
-    author_email="milosz.wasilewski@linaro.org",
-    license="GPL",
-    python_requires=">=3.5",
-    install_requires=["mkdocs>=1.1", "tags-macros-plugin"],
-    classifiers=[
-        "Development Status :: 5 - Production/Stable",
-        "Intended Audience :: Developers",
-        "Intended Audience :: Information Technology",
-        "License :: OSI Approved :: GNU General Public License v2 or later (GPLv2+)",
-        "Programming Language :: Python",
-        "Programming Language :: Python :: 3 :: Only",
-        "Programming Language :: Python :: 3.5",
-        "Programming Language :: Python :: 3.6",
-        "Programming Language :: Python :: 3.7",
-    ],
-    packages=find_packages(),
-    entry_points={
-        "mkdocs.plugins": [
-            "linaro-test-definitions = testdefinitionsmkdocs:LinaroTestDefinitionsMkDocsPlugin"
-        ]
-    },
-)