MNT mention #273 improvements in the changelog

[pierreglaser]

Pickling classes - try for example with this script:

importcloudpickleclassA:
passcloudpickle.dumps(A)

is now slightly faster (~10%) thanks to #273, so it is worth mentioning it in the changelog.

[pierreglaser]

(Rebased)

[codecov]

Codecov Report

Merging #277 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #277 +/- ##
=======================================
Coverage 91.58% 91.58% =======================================
Files 1 1 Lines 606 606 Branches 129 129 =======================================
Hits 555 555 Misses 31 31 Partials 20 20

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[ogrisel]

Thanks, merged.