Don't attempt to upgrade debug-info-tests to Python3.

There was CMake code that would attempt to detect Python 3 even if
LLVM was configured with Python 2, but it messed with the environment
for the other LLVM projects. This commit removes this functionality
and just lkeaves a hard error if Python < 3 is detected and
debuginfo-tests are required.

GitOrigin-RevId: d0416b91f0390f6e69dacf3d5d076531221e0767
1 file changed