[wip] fix: Namespace API

[charles-cooper]

What I did

fix some usability issues with Namespace

add `self` inside module analyser
refactor context manager implementation to use contextlib
change vyper namespace construction to use a classmethod
fix Namespace.__copy__()

TODO: there are some issues in the test suite with using a global namespace.

How I did it

How to verify it

Commit message

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Description for the changelog

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.20930% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 68.34%. Comparing base (7d466a6) to head (8b198f5).
Report is 4 commits behind head on master.

Files	Patch %	Lines
vyper/compiler/__init__.py	60.00%	2 Missing and 2 partials ⚠️
vyper/semantics/analysis/module.py	66.66%	2 Missing and 1 partial ⚠️
vyper/semantics/namespace.py	89.65%	2 Missing and 1 partial ⚠️
vyper/compiler/phases.py	97.14%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3799       +/-   ##
===========================================
- Coverage   85.22%   68.34%   -16.88%     
===========================================
  Files          92       92               
  Lines       13914    13946       +32     
  Branches     3118     3148       +30     
===========================================
- Hits        11858     9532     -2326     
- Misses       1561     3694     +2133     
- Partials      495      720      +225     

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