Fix inconsistency between traced files and reported files

[wjsi]

When solving #2776 which reported Plugin 'Cython.Coverage.Plugin' did not provide a file reporter for '/Users/wenjun.swj/miniconda3/lib/python3.7/site-packages/gevent/_hub_local.py' when executing coverage report, I find the cause is that some tracers built in Cython.Coverage do not have corresponding reporters.
In Cython.Coverage, when Plugin._parse_lines(c_file, filename) returns (None, None), Plugin.file_tracer(filename) returns a tracer, while Plugin.file_reporter(filename) returns None, and then coverage.py reports the error. This happens when shared packages have both *.py and *.c sharing the same base name. For instance, in the wheel package of gevent, both _hub_local.c and _hub_local.py exist, which mislead Cython.Coverage to produce a tracer as it does not ignore shared libraries. However, file_reporter() ignores shared libraries, and the report error is raised.
The simple solution is to ignore shared libraries in file_tracer like it does in file_reporter, and coverage report does not raise errors, thus fixes #2776 .

[scoder]

Thanks. Do you think you could come up with a test case for this? This seems like the kind of setup that will get lost and break right the next time we change the code.
You can look at the existing coverage .srctree tests in tests/run/, they are basically multiple files stuffed into one text archive, with the test commands at the top.

[wjsi]

@scoder A test case is added. It mocks the behavior of gevent._hub_local by importing a Cython-compiled accelerated version and reproduces the error under unmodified Cython.Coverage.

[scoder]

Is it actually necessary for this test to compile the .py files, in addition to the .pyx files?

I'm asking for two reasons: it complicates the test setup (thus increasing the risk of potentially unrelated failures), and there is currently a bug in your branch that I fixed in latest master regarding the compilation of __init__.py files under Windows, so I'm getting failures in appveyor. Merging master should resolve that, but if we can avoid this complication entirely, then that's clearly preferable.

[wjsi]

Done, changed to *.pyx.

[scoder]

Thanks