Update requirements.txt to reflect changes to setup.cfg

[cottsay]

These changes to the setup.cfg in colcon-core didn't get copied to this requirements file for bootstrapping colcon.

[Zyrin]

With this change the boorstrap process works now, but it does prints deprecation warnings:

setuptools/dist.py:759: SetuptoolsDeprecationWarning: 

License classifiers are deprecated.
!!

        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: OSI Approved :: Apache Software License

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!
  self._finalize_license_expression()
---

[Zyrin]

The boostrap instructions also contain the line pip install -U pip setuptools. Maybe that should also be removed.

[cottsay]

The boostrap instructions also contain the line pip install -U pip setuptools. Maybe that should also be removed.

Nice catch, fixed in 0b11d70.