Compatibility: expose cloudpickle.CloudPickler also as cloudpickle.Pickler

[twoertwein]

Some people/projects expect that a module implementing a Pickler, provides the Pickler class as <module>.Pickler, for example pytorch/pytorch#38098.

Since the current version of Cloudpickler doesn't define cloudpickle.Pickler, let's define cloudpickle.Pickler as cloudpickle.CloudPickler.

Fixes#366

[codecov]

Codecov Report

Merging #392 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #392 +/- ##
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Coverage 91.61% 91.61% =======================================
Files 3 3 Lines 656 656 Branches 135 135 =======================================
Hits 601 601 Misses 34 34 Partials 21 21 

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[pierreglaser]

LGTM, Thanks @twoertwein. This also needs a changelog entry - can you write one?
Regarding the README, issue, I think that at some point we should make it clear that accessing cloudpickle.Pickler is now the recommended way to use cloudpickle's Pickler subclass. But it is not urgent, so I won't mind if we leave the README as it is for now.

[pierreglaser]

Alright, let's merge! Thank you very much @twoertwein.

[pierreglaser]

I'll do a release of cloudpickle in the near future (to whom it may concern).