Bootstrap: update logic to support pre-and post python 3.8 (#8446)

PR #8373 (commit ID: 0193d5f) updated a `shutil.copytree(...)` command
to pass in the `dirs_exists_ok=True` argument. However, Python added
support for the `dirs_exists_ok` argument to `shutil.copytree` in 3.8.

Update the function call to inspect the python version, and only add the
argument if the script is running in a Python version equal, or higher
than 3.8.

rdar://147297864

Author: bkhouri

Utilities/bootstrap

@@ -592,7 +592,8 @@ def install_file(args, src, destination, destination_is_directory=True, ignored_
 
     logging.info("Installing %s to %s", src, dest)
     if os.path.isdir(src):
-        shutil.copytree(src, dest, ignore=shutil.ignore_patterns(*ignored_patterns), dirs_exist_ok=True)
+        additional_kwargs = { "dirs_exist_ok": True } if sys.version_info.major >=3 and sys.version_info.minor >= 8 else {}
+        shutil.copytree(src, dest, ignore=shutil.ignore_patterns(*ignored_patterns), **additional_kwargs)
     else:
         shutil.copy2(src, dest)