Add multi context

[drew-512]

Hi @sbinet

So this PR has all the py.Context stuff wrapped into it minus:

• examples/embedded
• examples/multi-context
• py/util.go (consumed by examples/embedded)
• README.md edits

On the sync.CloseSync issue you raised in the prev thread, the intention is to ensure that multiple holders of a py.Context can all call Close() without any (future) consequences. IMO, this is congruent with multiple holders of a py.Context (and how it's annoying when ya have to ensure that only one Close() is called). People see that close code and they can feel comforted it's an interface ready for the real world. And it basically comes at no cost or allocations.

The last commit on this PR should prevent any conflicts (assuming you merge PR 156 and 157 first)

Otherwise, lmk what you'd like to add/remove to this, otherwise I'll wait for this merge and then I'll immediately submit a PR with the above items rolled into a final PR or lmk what I should separate, etc. Thanks.

[codecov]

Codecov Report

Merging #158 (87d5755) into master (3068cb5) will decrease coverage by 0.17%.
The diff coverage is 66.12%.

@@ Coverage Diff @@## master #158 +/- ##
==========================================
- Coverage 74.55% 74.38% -0.18% 
==========================================
Files 64 66 +2 Lines 11187 11395 +208 ==========================================
+ Hits 8341 8476 +135 - Misses 2297 2352 +55 - Partials 549 567 +18 

Impacted Files	Coverage Δ
py/int.go	77.13% <0.00%> (+0.26%)	⬆️
py/traceback.go	22.22% <ø> (-2.11%)	⬇️
py/import.go	15.62% <45.16%> (-0.86%)	⬇️
modules/runtime.go	45.86% <45.86%> (ø)
py/run.go	60.46% <60.46%> (ø)
py/module.go	71.69% <72.50%> (+11.69%)	⬆️
pytest/pytest.go	49.09% <81.81%> (+1.92%)	⬆️
py/list.go	64.13% <83.33%> (+0.03%)	⬆️
sys/sys.go	60.75% <88.23%> (+1.60%)	⬆️
builtin/builtin.go	80.06% <88.88%> (+0.03%)	⬆️
... and 13 more

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[sbinet]

LGTM