Deprecation of tapir and the use of domains.TESTNET

[derekpierre]

Type of PR:

• Bugfix
• Feature
• Documentation
• Other

Required reviews:

• 1
• 2
• 3

What this does:
Deprecation of use of tapir testnet.

Issues fixed/closed:

• Fixes #...

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network. E.g.,
if this PR address a nucypher/productdev issue, let reviewers know!

Notes for reviewers:

What should reviewers focus on? Is there a particular commit/function/section
of your PR that requires more attention from reviewers?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (d90e3c6) to head (f4abc1d).
⚠️ Report is 1 commits behind head on epic-v0.7.x.

Additional details and impacted files

@@               Coverage Diff               @@
##           epic-v0.7.x     #785      +/-   ##
===============================================
- Coverage        90.29%   90.29%   -0.01%     
===============================================
  Files               98       98              
  Lines             8678     8676       -2     
  Branches           560      567       +7     
===============================================
- Hits              7836     7834       -2     
  Misses             799      799              
  Partials            43       43              

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