Clipping deformer & cosmetic fixes

[jatinkhilnani]

• Clipping deformer (Deformers to implement #1)
• Minor corrections to documentation

[bmcfee]

It looks like this PR only includes the cosmetic fixes?

[jatinkhilnani]

• Included the deformer code clipping.py with tests
• Have implemented the test parameterization both in the current and new syntax, that eliminates the warnings with pytest latest version (5.4.3)
• Added comments for test_deformers
• Open item: Per new syntax, added expectation (type of error) raised for xfail markers for Clipping method. The same is not working for LinearClipping & RandomClipping

[bmcfee]

A couple of minor cosmetic things, but this all looks good otherwise. Thanks!

[jatinkhilnani]

• Have also segregated and commented the deprecated marker syntax for better readability. Would we want these changes to be done for older tests too?
• Do I need to take care of the CI check failure, seems to be doing so for older Python versions. Also, please suggest if there are there any other documentation changes required for this before I move on to complete the other deformer.

[jatinkhilnani]

Along with changes per review comments, included the ones mentioned in comment above (not for older tests). Modified config to include pytest-cov==2.9.0, since v2.10.0 required pytest>=4.6 creating a conflict and causing build failure.

This does not completely resolve the CI check issues due to llvmlite>=0.31.0dev0->numba>=0.43.0->librosa>=0.4, as both the former packages are now compatible only with Python>=3.6. I see that librosa CI checks are now only performed for compatible Python versions. I suppose that needs to be modified here as well, or is there a workaround?

[jatinkhilnani]

Checked that the requested changes are applied. Please review and merge.

[bmcfee]

Thanks @jatinkhilnani ! I'll take the dependency issue in a separate thread, but yeah, we're generally overdue for some modernization.

[jatinkhilnani]

Thank you for the same. I have created another PR #82 for filter deformer, can include modernization (#81) changes in that one or contribute separately as well. Please suggest.