Docs: Add python3 to list of dependencies on some platforms

[danra]

python3 is required for running the unit tests on macOS.

Without python3 on macOS, 'make check' fails because /test/util/bitcoin-util-test.py fails to find python3.

[maflcko]

The dependencies for the functional tests are mentioned in https://github.com/danra/bitcoin/tree/master/test#dependencies

[danra]

@MarcoFalke OK, but I don't think it makes sense for build_osx.md to recommend running make check on one hand, while on the other hand not mentioning the dependencies for running it when listing the dependencies earlier.

[maflcko]

The readme in src/test mentions that the unit tests can be run with make check, but doesn't mention the python3 dependency. We might combine the src/test readme into the /test/ readme and then link to the combined readme from the /doc/ subdir.

[laanwj]

Concept ACK, as python 3 is required for the unit tests (not just the functional tests in test/functional) we should just mention it as dependency.
(it used to be the case that make check could be done without python 3)

[maflcko]

Fine with me, but please also update the dependencies for ubuntu/fedora

[danra]

@MarcoFalke I've only checked the dependencies on my macOS. Should I just add python3 to the build documentation for the platforms you mentioned as well?

[maflcko]

Yes, python3 will be required on the other platforms as well.

[danra]

@MarcoFalke Done

[maflcko]

utACK 7bb5d30

[jnewbery]

utACK 7bb5d30

[fanquake]

utACK 7bb5d30