Switch to depending on gnureadline instead

[cbrunnkvist]

The old readline package is only relevant up to Python<=3.3 and is now deprecated.

From https://pypi.org/project/readline/:

6.2.4.2 (2022-06-24)
THIS PACKAGE IS DEPRECATED! USE gnureadline INSTEAD!

The gnureadline package works/should work as a drop-in replacement.

Fixes installation errors such as:

      clang -bundle -undefined dynamic_lookup -Wl,-headerpad,0x1000 build/temp.macosx-10.9-universal2-cpython-39/Modules/3.x/readline.o readline/libreadline.a readline/libhistory.a -lncurses -o build/lib.macosx-10.9-universal2-cpython-39/readline.cpython-39-darwin.so -arch i386 -arch x86_64
      clang: error: no such file or directory: 'readline/libreadline.a'
      clang: error: no such file or directory: 'readline/libhistory.a'
      error: command '/usr/bin/clang' failed with exit code 1
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
  ERROR: Failed building wheel for readline
Failed to build readline
ERROR: Could not build wheels for readline, which is required to install pyproject.toml-based projects

[redfast00]

@swisskyrepo the install instructions in the readme currently don't work with Python 3.13.1 (which is the default on Arch Linux) because of the readline dependency not building any more. This PR fixes that.