Fix download package regardless of running python version

[ppiyakk2]

fixed: #5369

When the package described only support over python 3.5 and the python version of running pip is python 2.7, there will be occure an error during download package.

To fix this problem, I added an additional option 'ignore_requires_python' to ignore the requires python version when checking the support python version of package.

[cjerdonek]

@auvipy I think PR’s with behavior changes should have tests before they are marked approve.

In this case, I think a unit test should be included among the tests as CandidateEvaluator can now be tested easily in isolation.

[ppiyakk2]

@cjerdonek Thanks for your comment. I agreed with the this PR should have tests. But Actually I'm not good at writing test case, Could you give me some idea about test case?

check_requires_python() function is using sys.version_info to get a running python version.

I want to write testcase like below

1. If self._ignore_requires_python is False and required python version of package is 3.4
   -> then, If the running python version is python 2.7, the package will not be a candidate.
   -> else, the running python version is python 3.5, the package will be a candidate.

2. If self._ignore_requires_python is True, the package always be a candidate.

So, I think the test case can be affected by running python version, Is there any good idea to solve this problem?

[auvipy]

@auvipy I think PR’s with behavior changes should have tests before they are marked approve.

In this case, I think a unit test should be included among the tests as CandidateEvaluator can now be tested easily in isolation.

I do agree. I somehow missed that this PR doesn't include tests!! will be more careful!

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