[CMAKE] Use the new FindPython module

Summary:
The FindPythonInterp is replaced by FindPython since cmake 3.12.
This also solves a conflict with some pyenv installed versions (thanks @markblundeberg for reporting the issue !).

Test Plan:

ninja all bitcoin-bench check check-security check-functional

Run the OSX Gitian build.

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5284