build: specify dependencies directly in setup.cfg

In tox v4, "reuse of environments" was disabled [1]. This is then later
explained [2] to refer to exactly that thing which we were using for
inheriting the dependencies from the top-level testenv all the way to
the docs build. That's why the docs build in GitHub CI started failing.

IMHO, The Correct Way™ of specifying what dependencies are used for
which feature are the so-called "extra dependencies". Once they are in
place, it is be possible to install gnpy via, e.g., `pip install
oopt-gnpy[docs]`. However, this process is (as far as I can tell)
incompatible with `requirements.txt`; all my attempts at using the
standard dependency syntax in that file have failed for me.

So, in order to make this happen, let's move all the dependencies from a
more-or-less ad-hoc collection of files to this declarative approach
right in setup.cfg. That way, the deps are listed on a single place, in
a declarative manner, and as a result, the installation is now a trivial
pip oneliner.

As a result, one can also remove that duplication of dependencies in
docs requirements.

[1] https://tox.wiki/en/4.11.4/upgrading.html#reuse-of-environments
[2] https://tox.wiki/en/4.11.4/upgrading.html#packaging-configuration-and-inheritance

Change-Id: I34aa0c71e993b39e2b805a7de40e133b4d290318
Fixes: 47c89626 fix docs requirements

setup.cfg

@@ -49,3 +49,35 @@ console_scripts =
     gnpy-transmission-example = gnpy.tools.cli_examples:transmission_main_example
     gnpy-path-request = gnpy.tools.cli_examples:path_requests_run
     gnpy-convert-xls = gnpy.tools.convert:_do_convert
+
+[options]
+install_requires =
+	# matplotlib 3.8 removed support for Python 3.8
+	matplotlib>=3.7.3,<4
+	# networkx 3.2 removed support for Python 3.8
+	networkx>=3.1,<4
+	# numpy 1.25 removed support for Python 3.8
+	numpy>=1.24.4,<2
+	pbr>=6.0.0,<7
+	# scipy 1.11 removed support for Python 3.8
+	scipy>=1.10.1,<2
+	# xlrd 2.x removed support for .xlsx, it's only .xls now
+	xlrd>=1.2.0,<2
+
+[options.extras_require]
+tests =
+	build>=1.0.3,<2
+	pytest>=7.4.3,<8
+	# pandas 2.1 removed support for Python 3.8
+	pandas>=2.0.3,<3
+	# flake v6 killed the --diff option
+	flake8>=5.0.4,<6
+
+docs =
+	alabaster>=0.7.12,<1
+	docutils>=0.17.1,<1
+	myst-parser>=0.16.1,<1
+	Pygments>=2.11.2,<3
+	rstcheck
+	Sphinx>=5.3.0,<6
+	sphinxcontrib-bibtex>=2.4.1,<3