buildman: Fix full help for Python 3.8

With Python versions older than 3.9 Buildman produces an error on
start-up. Fix this with a workaround for importlib.

There is already a workaround for v3.6 but I am not sure if that is still
functioning.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/main.py b/tools/buildman/main.py
index 5f42a58..3cf877e 100755
--- a/tools/buildman/main.py
+++ b/tools/buildman/main.py
@@ -7,7 +7,7 @@
 """See README for more information"""
 
 try:
-    from importlib.resources import files
+    import importlib.resources
 except ImportError:
     # for Python 3.6
     import importlib_resources
@@ -83,7 +83,13 @@
         run_test_coverage()
 
     elif args.full_help:
-        tools.print_full_help(str(files('buildman').joinpath('README.rst')))
+        if hasattr(importlib.resources, 'files'):
+            dirpath = importlib.resources.files('buildman')
+            tools.print_full_help(str(dirpath.joinpath('README.rst')))
+        else:
+            with importlib.resources.path('buildman', 'README.rst') as readme:
+                tools.print_full_help(str(readme))
+
 
     # Build selected commits for selected boards
     else: