Move the most of metadata into setup.cfg.

Add `pyproject.toml` according to PEP 517.

setup.cfg

@@ -1,3 +1,86 @@
+[metadata]
+name = GPy
+author_email = gpy.authors@gmail.com
+license = BSD-3-Clause
+description = The Gaussian Process Toolbox
+keywords =
+    machine-learning
+    gaussian-processes
+    kernels
+url = https://sheffieldml.github.io/GPy/
+download_url = https://github.com/SheffieldML/GPy/archive/refs/heads/devel.zip
+long_description = file: README.rst
+classifiers =
+    License :: OSI Approved :: BSD License
+    Natural Language :: English
+    Operating System :: MacOS :: MacOS X
+    Operating System :: Microsoft :: Windows
+    Operating System :: POSIX :: Linux
+    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
+    Framework :: IPython
+    Intended Audience :: Science/Research
+    Intended Audience :: Developers
+    Topic :: Software Development
+    Topic :: Software Development :: Libraries :: Python Modules
+project_urls =
+    Source Code = https://github.com/SheffieldML/GPy
+    Bug Tracker = https://github.com/SheffieldML/GPy/issues
+
+[options]
+package_dir =
+    GPy = GPy
+py_modules = GPy.__init__
+packages =
+    GPy
+    GPy.core
+    GPy.core.parameterization
+    GPy.kern
+    GPy.kern.src
+    GPy.kern.src.psi_comp
+    GPy.models
+    GPy.inference
+    GPy.inference.optimization
+    GPy.inference.mcmc
+    GPy.inference.latent_function_inference
+    GPy.likelihoods
+    GPy.mappings
+    GPy.examples
+    GPy.testing
+    GPy.util
+    GPy.plotting
+    GPy.plotting.gpy_plot
+    GPy.plotting.matplot_dep
+    GPy.plotting.matplot_dep.controllers
+    GPy.plotting.plotly_dep
+setup_requires = numpy>=1.7
+install_requires =
+    numpy>=1.7
+    six
+    paramz>=0.9.0
+    cython>=0.29
+    scipy>=1.3.0; python_version >= '3.6'
+    scipy>=1.3.0,<1.5.0; python_version < '3.6'
+include_package_data = True
+test_suite = GPy.testing
+
+[options.extras_require]
+docs = sphinx
+optional = mpi4py; ipython>=4.0.0
+# matplotlib Version see github issue #955
+plotting = 
+    matplotlib==3.3.4; python_version >= '3.6'
+    matplotlib==3.0.0; python_version < '3.6'
+    plotly >= 1.8.6
+notebook =
+    jupyter_client >= 4.0.6
+    ipywidgets >= 4.0.3
+    ipykernel >= 4.1.0
+    notebook >= 4.0.5
+
 [bumpversion]
 current_version = 1.9.9
 tag = True
@@ -9,7 +92,3 @@ commit = True
 
 [upload_docs]
 upload-dir = doc/build/html
-
-[medatdata]
-description-file = README.rst
-