payjoin-ffi README fixes

[spacebear21]

Consolidate the test sections and update the repository URL.

[DanGould]

pip install --requirement requirements.txt --requirement requirements-dev.txt

This is still in here, and is wrong because requirements.txt no longer exists

nvm, was in the wrong directory

[DanGould]

tACK 939f204

followed on local and it appears to work

[coveralls]

Pull Request Test Coverage Report for Build 15425258167

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.329%

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Totals
Change from base Build 15400978156:	0.0%
Covered Lines:	6420
Relevant Lines:	7613

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💛 - Coveralls