Remove setup.py

This finalizes the switch from the deprecated setup.py to
pyproject.toml.  Setuptools is still used internally but not on the
command line.  Use pip to install khard or build the version.py file.

See also pep 517 and 518 or
https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html.

.github/workflows/ci.yml

@@ -46,11 +46,11 @@ jobs:
     - uses: actions/checkout@v3
     - name: Set up Python
       uses: actions/setup-python@v4
+      with:
+        python-version: "3.12"
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
         python -m pip install .[doc]
     - name: Build the documentation
-      run: |
-        python setup.py build
-        make -C doc html man
+      run: make -C doc html man

.readthedocs.yml

@@ -13,10 +13,6 @@ sphinx:
 
 python:
   install:
-    # Run setup.py first to generate the version.py file. For some reason it
-    # is not generated with `pip install .[doc]`
-    - method: setuptools
-      path: .
     - method: pip
       path: .
       extra_requirements: