[rebranch] Drop minimum python support to 3.6

This was changed back in Februrary, but we still support platforms that
only have Python 3.6 (Amazon Linux 2).

(cherry picked from commit ad29658)

Author: bnbarham

llvm/CMakeLists.txt

@@ -1034,7 +1034,7 @@ set(LLVM_PROFDATA_FILE "" CACHE FILEPATH
 
 if(LLVM_INCLUDE_TESTS)
   # All LLVM Python files should be compatible down to this minimum version.
-  set(LLVM_MINIMUM_PYTHON_VERSION 3.8)
+  set(LLVM_MINIMUM_PYTHON_VERSION 3.6)
 else()
   # FIXME: it is unknown if this is the actual minimum bound
   set(LLVM_MINIMUM_PYTHON_VERSION 3.0)