[lldb] Remove Python 2 fallback and only support Python 3

This removes the fallback to Python 2 and makes Python 3 the only
supported configuration. This is the first step to fully migrate to
Python 3 over the coming releases as discussed on the mailing list.

http://lists.llvm.org/pipermail/lldb-dev/2020-August/016388.html

As a reminder, for the current release the test suite and the generated
bindings should remain compatible with Python 2.

Differential revision: https://reviews.llvm.org/D85942

(cherry picked from commit ce439cb)

Author: JDevlieghere

lldb/cmake/modules/FindPythonInterpAndLibs.cmake

@@ -38,41 +38,12 @@ macro(FindPython3)
   endif()
 endmacro()
 
-macro(FindPython2)
-  # Use PYTHON_HOME as a hint to find Python 2.
-  set(Python2_ROOT_DIR "${PYTHON_HOME}")
-  find_package(Python2 COMPONENTS Interpreter Development)
-  if(Python2_FOUND AND Python2_Interpreter_FOUND)
-    set(PYTHON_LIBRARIES ${Python2_LIBRARIES})
-    set(PYTHON_INCLUDE_DIRS ${Python2_INCLUDE_DIRS})
-    set(PYTHON_EXECUTABLE ${Python2_EXECUTABLE})
-
-    set(PYTHON2_FOUND TRUE)
-    mark_as_advanced(
-      PYTHON_LIBRARIES
-      PYTHON_INCLUDE_DIRS
-      PYTHON_EXECUTABLE
-      SWIG_EXECUTABLE)
-  endif()
-endmacro()
-
 if(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS AND PYTHON_EXECUTABLE AND SWIG_EXECUTABLE)
   set(PYTHONINTERPANDLIBS_FOUND TRUE)
 else()
   find_package(SWIG 2.0)
   if (SWIG_FOUND)
-    if (LLDB_PYTHON_VERSION)
-      if (LLDB_PYTHON_VERSION VERSION_EQUAL "2")
-        FindPython2()
-      elseif(LLDB_PYTHON_VERSION VERSION_EQUAL "3")
-        FindPython3()
-      endif()
-    else()
       FindPython3()
-      if (NOT PYTHON3_FOUND AND NOT CMAKE_SYSTEM_NAME STREQUAL Windows)
-        FindPython2()
-      endif()
-    endif()
   else()
     message(STATUS "SWIG 2 or later is required for Python support in LLDB but could not be found")
   endif()