fix python executable version in unicorn-emulate

[theguy147]

fix python executable version in unicorn-emulate

Description/Motivation/Screenshots

Changes which("python") to which("python3") in the UnicornEmulateCommand.

This is required on some Distros to work correctly.

How Has This Been Tested?

Architecture	Yes/No	Comments
x86-32	✖️
x86-64	✔️	quick manual testing
ARM	✖️
AARCH64	✖️
MIPS	✖️
POWERPC	✖️
SPARC	✖️
RISC-V	✖️
make tests	✔️	note: it is actually make test and not make tests

Checklist

• My PR was done against the dev branch, not master.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required. (not required!)
• My change adds tests as appropriate. (not needed)
• I have read and agree to the CONTRIBUTING document.

EDIT:
referencing #677

[theguy147]

Seeing as python2 is anyway no longer supported since gef release 2020.03 I would also suggest to remove the pip2 installation instructions for the optional dependencies in the documentation (in docs/index.md).

I will add this change to this PR

[Grazfather]

LGTM.

Need to test it out.