Build instructions need wheel package

[simonjwright]

For macOS, Python 3.8.6, the instructions in /user_manual/building.rst should tell us to install the python wheel package into a new venv before building Langkit.

If wheel isn’t installed, when you attempt to reinstall Langkit you get

Using legacy 'setup.py install' for Langkit, since package 'wheel' is not installed.
Installing collected packages: Langkit
  Attempting uninstall: Langkit
    Found existing installation: Langkit 0.1.dev0
ERROR: Cannot uninstall 'Langkit'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.

[pmderodat]

Hello @simonjwright,

Thank you for reporting this. We’ll add wheel to the REQUIREMENTS.dev file, used to install Python dependencies before installing Langkit in the build instructions. Please let us know if this is not enough.