[SR-7320] GYB is run with python, not python2.7

[swift-ci]

Previous ID	SR-7320
Radar	None
Original Reporter	sanxiyn (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug, CMake, StarterBug
Assignee	None
Priority	Medium

md5: a456e83bcda5542ee8cf2382f8fa9b21

Issue Description:

#1535 made GYB run with python2.7, but #6463 effectively reverted the change because GYB is run with PYTHON_EXECUTABLE instead of using shebang line. This causes the problem 1535 fixed to reappear.

[belkadan]

Sounds like the way to fix this is to change the following line in the top-level CMakeLists.txt:

find_package(PythonInterp REQUIRED)

to

find_package(PythonInterp 2.7 REQUIRED)
if(…) # check if 2.7 wasn't found
  find_package(PythonInterp 3 REQUIRED)
endif()

and then test it, because I just wrote that here without trying it myself.