llvm's CMakeLists has a Special case to reject python versions != 2.7 due to a dependency on that version by lldb that is no longer required.
224948 was introduced to workaround this limitation in lldb. However it appears this is no needed, as lldb builds with python >= 2.7 since @zturner made improvements to the lldb's cmake in 249466, supporting python3 in lldb. Additionally, python 3.5 is required for Visual Studio 2015 debug builds with python bindings, so the reasons for this limitation seem obsolete.
I'm not sure whether there are other repercussions from disabling this check, so I'd appreciate comments with those more familiar with build system