Remove annotations for python3.6 and below

[cicdw]

Closes#193 and closes#196 with a heavy hand, by removing the ability to include __annotations__ in versions of python < 3.7.

It seemed the ability to pickle arbitrary typing classes was a deep rabbit hole external to cloudpickle, and AFAIK the alternative would be some sort of check / try / except on annotations that would lead to inconsistent pickling of __annotations__.

[ogrisel]

I have no strong opinion. I believe it's ok to remove a half broken half working feature. We should probably do it as part of 0.6 and document that well in the changelog.

[ogrisel]

I pinged several reviewers to get a second opinion on this change.

[pitrou]

Agreed with this. It's unfortunate that typing was designed in such a baroque and unusual way. Not the greatest Python feature IMO :-/

[mrocklin]

No objection from me. I don't know this topic well though. I'm inclined to trust @pitrou .

[ogrisel]

@cicdw can you please add an entry to document this change to the change log file?

[cicdw]

@ogrisel yup, done.

[codecov-io]

Codecov Report

Merging #197 into master will not change coverage.
The diff coverage is 100%.

@@ Coverage Diff @@## master #197 +/- ##
=======================================
Coverage 85.16% 85.16% =======================================
Files 1 1 Lines 573 573 Branches 111 111 =======================================
Hits 488 488 Misses 62 62 Partials 23 23

Impacted Files	Coverage Δ
cloudpickle/cloudpickle.py	85.16% <100%> (ø)	⬆️

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[ogrisel]

Alright, let's merge then! Thanks @cicdw.