FIX python-lz4 version comparison

[sveitser]

The python-lz4 package has its version string prefixed with "v". As a result on nixos a test fails.

This commit changes to pkg_resources.parse_version which normalizes
the two schemas correctly and seems to be the recommended way to
compare versions in python at the moment.

Nixos build log https://hydra.nixos.org/build/88820470/nixlog/2.

Failing test:

=================================== FAILURES ===================================
_____________________________ test_lz4_compression _____________________________

tmpdir = local('/build/pytest-of-nixbld/pytest-0/test_lz4_compression0')

    @with_lz4
    def test_lz4_compression(tmpdir):
        # Check that lz4 can be used when dependency is available.
        import lz4.frame
        compressor = 'lz4'
        assert compressor in _COMPRESSORS
        assert _COMPRESSORS[compressor].fileobj_factory == lz4.frame.LZ4FrameFile
    
        fname = tmpdir.join('test.pkl').strpath
        data = 'test data'
>       numpy_pickle.dump(data, fname, compress=compressor)

joblib/test/test_numpy_pickle.py:1047: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
joblib/numpy_pickle.py:498: in dump
    compress_level)) as f:
joblib/numpy_pickle_utils.py:192: in _write_fileobject
    filename, compresslevel=compresslevel)
joblib/compressor.py:234: in compressor_file
    self._check_versions()
joblib/compressor.py:229: in _check_versions
    if lz4 is None or LooseVersion(lz4.__version__) < LooseVersion('0.19'):
/nix/store/6la3fblc497pacyy0vxhnjq1l1wlb5w7-python3-3.7.2/lib/python3.7/distutils/version.py:52: in __lt__
    c = self._cmp(other)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = LooseVersion ('v2.1.2'), other = LooseVersion ('0.19')

    def _cmp (self, other):
        if isinstance(other, str):
            other = LooseVersion(other)
    
        if self.version == other.version:
            return 0
>       if self.version < other.version:
E       TypeError: '<' not supported between instances of 'str' and 'int'

[codecov]

Codecov Report

Merging #847 into master will decrease coverage by 1.91%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #847      +/-   ##
==========================================
- Coverage   94.55%   92.63%   -1.92%     
==========================================
  Files          45       45              
  Lines        6407     6407              
==========================================
- Hits         6058     5935     -123     
- Misses        349      472     +123

Impacted Files	Coverage Δ
joblib/compressor.py	90.93% <100%> (-2.59%)	⬇️
joblib/backports.py	93.75% <100%> (+54.16%)	⬆️
joblib/numpy_pickle_compat.py	49% <0%> (-42%)	⬇️
joblib/test/test_numpy_pickle.py	88.5% <0%> (-9.86%)	⬇️
joblib/testing.py	87.5% <0%> (-7.5%)	⬇️
joblib/_dask.py	91.52% <0%> (-3.96%)	⬇️
joblib/numpy_pickle.py	94.58% <0%> (-3.95%)	⬇️
joblib/_multiprocessing_helpers.py	76.92% <0%> (-3.85%)	⬇️
joblib/numpy_pickle_utils.py	90.32% <0%> (-3.23%)	⬇️
... and 15 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #847 into master will increase coverage by 11.2%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #847      +/-   ##
==========================================
+ Coverage   84.15%   95.36%   +11.2%     
==========================================
  Files          45       45              
  Lines        6406     6407       +1     
==========================================
+ Hits         5391     6110     +719     
+ Misses       1015      297     -718

Impacted Files	Coverage Δ
joblib/compressor.py	93.52% <100%> (+11%)	⬆️
joblib/backports.py	93.75% <100%> (+58.33%)	⬆️
joblib/test/test_memmapping.py	99.41% <0%> (+0.29%)	⬆️
joblib/_parallel_backends.py	93.6% <0%> (+0.79%)	⬆️
joblib/memory.py	95.69% <0%> (+1.34%)	⬆️
joblib/test/test_hashing.py	98.88% <0%> (+1.67%)	⬆️
joblib/logger.py	86.84% <0%> (+2.63%)	⬆️
joblib/parallel.py	97.28% <0%> (+2.71%)	⬆️
joblib/_memmapping_reducer.py	95% <0%> (+2.77%)	⬆️
joblib/my_exceptions.py	98.11% <0%> (+3.77%)	⬆️
... and 22 more

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[FlorianFranzen]

Would be nice if we can get this merged.

[aabadie]

LGTM

@ogrisel any idea why the scikit-learn tests are failing ? I tried with 0.20.3 on my fork but have another failing test, see https://travis-ci.org/aabadie/joblib/jobs/507830463 and aabadie@17b3ee3
This is also broken on master btw.

[ogrisel]

I merged @lesteve's #861 to master and merged master into this branch to check whether that fixes the problem.

[ogrisel]

The windows error is unrelated, let's merge.