Fails to identify package that specifies python_requires.

[rotu]

If you use a package that specifies a python version, colcon_setup_py chokes. This is because we assume that repr and ast.literal_eval are inverse operations. This is not guaranteed, and in fact not true for SpecifierSet:

$ colcon info --packages-select tox
/usr/lib/python3.8/distutils/dist.py:274: UserWarning: Unknown distribution option: 'use_scm_version'
  warnings.warn(msg)
[1.315s] ERROR:colcon.colcon_core.package_identification:Exception in package identification extension 'python_setup_py' in 'src/tox': invalid syntax (<unknown>, line 1)
Traceback (most recent call last):
  File "/home/dan/Documents/rostools/src/colcon-core/colcon_core/package_identification/__init__.py", line 143, in _identify
    retval = extension.identify(_reused_descriptor_instance)
  File "/home/dan/Documents/rostools/src/colcon-python-setup-py/colcon_python_setup_py/package_identification/python_setup_py.py", line 47, in identify
    config = get_setup_information(setup_py)
  File "/home/dan/Documents/rostools/src/colcon-python-setup-py/colcon_python_setup_py/package_identification/python_setup_py.py", line 267, in get_setup_information
    _setup_information_cache[hashable_env] = _get_setup_information(
  File "/home/dan/Documents/rostools/src/colcon-python-setup-py/colcon_python_setup_py/package_identification/python_setup_py.py", line 309, in _get_setup_information
    return ast.literal_eval(output)
  File "/usr/lib/python3.8/ast.py", line 59, in literal_eval
    node_or_string = parse(node_or_string, mode='eval')
  File "/usr/lib/python3.8/ast.py", line 47, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 1
    {'package_data': {}, 

...

 'python_requires': <SpecifierSet('!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7')>}}
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ^
SyntaxError: invalid syntax

[1.860s] WARNING:colcon.colcon_core.package_selection:ignoring unknown package 'tox' in --packages-select

[XYudong]

Hi, I got similar error as "ERROR:colcon.colcon_core.package_identification:Exception in package identification extension 'python_setup_py' in 'my_package': invalid syntax (, line 1)". Do you have any clue to this?