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 (55d7dc7) to head (f4abc1d).
⚠️ Report is 333 commits behind head on epic-v0.7.x.

Additional details and impacted files

@@               Coverage Diff               @@
##           epic-v0.7.x     #785      +/-   ##
===============================================
+ Coverage        87.45%   90.29%   +2.84%     
===============================================
  Files               82       98      +16     
  Lines             5778     8676    +2898     
  Branches           391      567     +176     
===============================================
+ Hits              5053     7834    +2781     
- Misses             688      799     +111     
- Partials            37       43       +6     

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