fix(build.site) setup.py build_sphinx is deprecated >

from main *sphinx* project, and must opt-in with `[setuptools]` extras,
as described in:
  - sphinx-doc/sphinx#9595

An alternative is to execute the `sphinx-build` cmd,
as described in the [docs](https://www.sphinx-doc.org/en/master/usage/quickstart.html):

  sphinx-build -b html docs/source/ docs/build/

or

  cd docs && make html

Author: ankostis

setup.py

@@ -47,7 +47,7 @@ def _ask_git_version(default: str) -> str:
 ]
 matplot_deps = plot_deps + ["matplotlib"]
 sphinx_deps = plot_deps + [
-    "sphinx >=2",
+    "sphinx[setuptools] >=2",
 ]
 test_deps = list(
     set(