Outdated documentation for dev environment

[hodgesmr]

The developer dependency documentation appears to be out of date:

https://github.com/Axelrod-Python/Axelrod/blob/3fc68ed2af39b812a7db601f76c7f75228f187bb/docs/how-to/contributing/setting_up_the_environment.rst?plain=1#L7-L17

The file requirements/development.txt doesn't appear to exist. Looks like it was removed in #1440