Switch to built-in setuptools functionality

[lemenkov]

Looks like setuptools-markdown is deprecated in favor of a built-in setuptools functionality. Worse is that setuptools_markdown isn't compatible with a recent versions of a pypandoc library. Let's just drop it.

What I did

Related issue: man-group/pytest-plugins#87

How I did it

Stumbled upon this message while building:

/usr/bin/python3 setup.py build '--executable=/usr/bin/python3 -sP'
/usr/lib/python3.11/site-packages/setuptools/installer.py:27: SetuptoolsDeprecationWarning: setuptools.installer is deprecated. Requirements should be satisfied by a PEP 517 installer.
  warnings.warn(
Traceback (most recent call last):
  File "/home/petro/rpmbuild/BUILD/py-solc-x-1.1.1/setup.py", line 5, in <module>
    setup(
  File "/usr/lib/python3.11/site-packages/setuptools/__init__.py", line 87, in setup
    return distutils.core.setup(**attrs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/setuptools/_distutils/core.py", line 139, in setup
    _setup_distribution = dist = klass(attrs)
                                 ^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/setuptools/dist.py", line 477, in __init__
    _Distribution.__init__(
  File "/usr/lib/python3.11/site-packages/setuptools/_distutils/dist.py", line 275, in __init__
    self.finalize_options()
  File "/usr/lib/python3.11/site-packages/setuptools/dist.py", line 901, in finalize_options
    ep(self)
  File "/usr/lib/python3.11/site-packages/setuptools/dist.py", line 922, in _finalize_setup_keywords
    ep.load()(self, ep.name, value)
  File "/home/petro/rpmbuild/BUILD/py-solc-x-1.1.1/.eggs/setuptools_markdown-0.4.1-py3.11.egg/setuptools_markdown.py", line 43, in long_description_markdown_filename
    output = pypandoc.convert(markdown_filename, 'rst', format='md')
             ^^^^^^^^^^^^^^^^
AttributeError: module 'pypandoc' has no attribute 'convert'

How to verify it

Run with a recent pypandoc. It will throw something like "AttributeError: module 'pypandoc' has no attribute 'convert'"

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation (README.md)
• I have added an entry to the changelog

[antazoey]

Completed in #161 as part of the general Ape-ifications