configure: work around ImportError: cannot import name 'main'

Author: cirosantilli

configure

@@ -100,7 +100,9 @@ if "$qemu"; then
 fi
 if "$gem5"; then
   # Generate graphs of config.ini under m5out.
-  pip install --user pydot
+  # Not with pip directly:
+  # https://stackoverflow.com/questions/49836676/error-after-upgrading-pip-cannot-import-name-main/51846054#51846054
+  python -m pip install --user pydot
 fi
 
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