improve capture regex in setup.py cataloger

As pointed out in PR https://github.com/anchore/syft/pull/150

In the Python `setup.py` cataloger, there is an opportunity to be more lenient with spaces and capture more environment marker cases with `['"]\s*([\w]+\s*==\s*[\w\.]*)\s*[;'"]` which would find `ipaddress==1.0` in:
```
setup(
...
install_requires=[
'six',
'humanize',
],
extras_require={
':python_version == "2.7"': [
'ipaddress==1.0',
],
},
)
```

Start with the Python setup.py cataloger and review PR #150 for the existing capture behavior. Check the regex against the setup.py example in this issue, including spaces and the python_version marker; done means the dependency ipaddress==1.0 is captured without regressing ordinary requirements.

Written by the indexing model from the issue text.