setuptools min version

[dirk-thomas]

Fixes #90.

The first commit declares the minimum version of setuptools required. This only works when using pip to install the package. The version dependency can't be specified in the stdeb.cfg file since some supported platforms like Ubuntu Xenial don't provide a new enough Debian package of setuptools.

The second commit will catch the case where the available setuptools version is too old and provide a custom error message describing the problem as well as a way to update the package via pip.

[sloretz]

LGTM

Tested by first making a wheel in a python3 venv with setuptools 40

(env) $ cd colcon-core
(env) $ python setup.py bdist_wheel
...
(env) $ deactivate

Then in a new venv with setuptools 20.7.0

(renv) $ pip install src/colcon-core/dist/colcon_core-0.3.5-py3-none-any.whl
Processing ./src/colcon-core/dist/colcon_core-0.3.5-py3-none-any.whl
...
Collecting setuptools>=30.3.0 (from colcon-core==0.3.5)
  Using cached https://files.pythonhosted.org/packages/...
...
(renv) $ python -c "import setuptools; print(setuptools.__version__)"
40.0.0

[sloretz]

sys.exit() is kind of harsh for an ImportError, but it does make the message easy to read when trying to bootstrap without installing setuptools.

$ ./src/colcon-core/bin/colcon
'setuptools' needs to be at least version 30.3.0, if a newer version is not available from the package manager use 'pip3 install -U setuptools' to update to the latest version

I'm ok with it, but would prefer a raised ImportError instead.

[dirk-thomas]

I modified the patch to extend the raised ModuleNotFoundError exception with additional information where applicable: 8a71269#diff-9fc4dc6d196841f3b396afba788b9246R18

The "downside" is that the build will continue, likely falling back to the python_setup_py extension which fails for every package using a setup.cfg file rather than declaring the package name in the setup.py file.

[dirk-thomas]

ModuleNotFoundError is actually only available as of Pytohn 3.6. Therefore I updated the patch to catch the more generic exception ImportError. That should make the CI with Python 3.5 happy too.

[codecov-io]

Codecov Report

Merging #91 into master will decrease coverage by 0.21%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   78.25%   78.03%   -0.22%     
==========================================
  Files          50       50              
  Lines        2768     2777       +9     
  Branches      462      463       +1     
==========================================
+ Hits         2166     2167       +1     
- Misses        580      588       +8     
  Partials       22       22

Impacted Files	Coverage Δ
colcon_core/package_identification/python.py	85.45% <20%> (-14.55%)	⬇️

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