Modernize build system to use setup.cfg

For now, keep but deprecate the requirements/*.txt files.

docs/batch.rst

@@ -111,7 +111,7 @@ Users may need to customize the script to meet their requirements.
 
 .. code-block:: bash
 
-    pip3 install -r requirements/watcher.txt
+    pip3 install ocrmypdf[watcher]
 
     env OCR_INPUT_DIRECTORY=/mnt/input-pdfs \
         OCR_OUTPUT_DIRECTORY=/mnt/output-pdfs \

docs/installation.rst

@@ -812,8 +812,7 @@ To install all of the development and test requirements:
     python3 -m venv
     source venv/bin/activate
     cd OCRmyPDF
-    pip install -e .
-    pip install -r requirements/dev.txt -r requirements/test.txt
+    pip install -e .[test]
 
 To add JBIG2 encoding, see :ref:`jbig2`.