Deprecate local_context() and update contexts.rst

[skirpichev]

• use context() instead of local_context() in the tutorial
• drop "Comments on the ieee context" section
• drop "Nested contexts and coding style"
• deprecate local_context() and update context() docstring, closes #454
• drop CTXT_Manager_Type
• update 1st paragraph of docs/contexts.rst
• change local_context() -> context() in tests and docs
• slightly expand tutorial.rst and drop "Contexts and the with statement" section
• improve test coverage for GMPy_CTXT_Context()

[skirpichev]

@casevh, this should address #454.

Missing parts: 1) removal of CTXT_Manager_Type 2) change local_context -> context in tests 3) "Contexts and the with statement" section (probably should be part of the tutorial) 4) 1st paragraph of contexts.rst

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2306188) 85.41% compared to head (7a40966) 85.51%. Report is 1 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #460 +/- ## ========================================== + Coverage 85.41% 85.51% +0.09% ========================================== Files 49 49 Lines 11727 11690 -37 Branches 2211 2211 ========================================== - Hits 10017 9997 -20 + Misses 1710 1693 -17 ```

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[skirpichev]

This is ready for review.

[casevh]

Thanks for the cleanup. I'll merge it later this evening.

[skirpichev]

Hmm, there are some uncovered lines. I'll add more tests.

[skirpichev]

Ok, there is no regressions in tests.