Revert type hints support

[pierreglaser]

Following #298 (comment), we need to revert the support for pickling annotation in Python3.4-3.6 introduced in #276 . Pickling annotations on those versions was a known issue (#193) , but I was not aware of it when #276 got merged.
I added a note in the readme, as well as some comments in the code to make it clear why we start supporting annotations from 3.7 whereas they were introduced in python3.4.

[codecov]

Codecov Report

Merging #299 into master will decrease coverage by 25.35%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #299 +/- ##
===========================================
- Coverage 92.84% 67.48% -25.36% 
===========================================
Files 2 2 Lines 852 852 Branches 177 177 ===========================================
- Hits 791 575 -216 - Misses 37 257 +220 + Partials 24 20 -4

Impacted Files	Coverage Δ
cloudpickle/cloudpickle.py	92% <100%> (ø)	⬆️
cloudpickle/cloudpickle_fast.py	0% <0%> (-95.16%)	⬇️

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[codecov]

Codecov Report

Merging #299 into master will not change coverage.
The diff coverage is 100%.

@@ Coverage Diff @@## master #299 +/- ##
=======================================
Coverage 92.84% 92.84% =======================================
Files 2 2 Lines 852 852 Branches 177 177 =======================================
Hits 791 791 Misses 37 37 Partials 24 24

Impacted Files	Coverage Δ
cloudpickle/cloudpickle.py	92% <100%> (ø)	⬆️

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[jrbourbeau]

Thanks for your work on this @pierreglaser! I just wanted to gently check in on this PR. It look like the test failures here are unrelated to the changes in this PR and I think were fixed in #300 (by you in fact!)

[pierreglaser]

I'm going to merge soon, but maybe @ogrisel or @llllllllll could make a pass before (should be trivial)

[llllllllll]

LGTM

[ogrisel]

Some nitpicks both otherwise LGTM.

[ogrisel]

Merged. Thanks @pierreglaser!

[jrbourbeau]

Thanks @pierreglaser! This should resolve an issue we're seeing over in dask/dask#5317

[ksangeek]

Thanks @pierreglaser for fixing this and @jrbourbeau for identifying this as the root cause for dask/dask#5317!

@pierreglaser@ogrisel May I request a new release for this fix to be picked up in released conda packages - https://anaconda.org/search?q=cloudpickle.