Pieter Wuille authored Apr 15, 2019 and freetrader committed May 25, 2020

Summary:
```
This removes the dependency on OpenSSL for the interaction tests, by
providing a pure-Python
toy implementation of secp256k1.
```

Backport of core [[https://github.com/bitcoin/bitcoin/pull/15826 | PR15826]].

Test Plan:
  ninja check-functional-extended

*The following depends on D14007:*
Build for windows, then:
 - Install python if needed and add it to your path
 - If you copy the project to some place, you need at least the `src`,
`test`, `share` and your build directories.
 - Copy the test_runner.py to the build directory to replace the symlink
 - Update the config.ini file to match you paths
Then in a powershell:
  $env:PYTHONIOENCODING="utf-8"
  python .\test\functional\test_runner.py --force
You should note that only a few tests remain red, in particular all the
schnorr related tests since they still use openssl.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5965