Support Python 3.8.10.

Redshell unfortunately uses this old version, while the union operator
for dictionaries was only added in Python 3.9.

Test: Ran cargo2android.py on a crate with variants
Change-Id: I7a875acde11c23a3b789ef384f3ab32100d7c6d5

scripts/cargo2android.py

@@ -1339,7 +1339,7 @@ class Runner(object):
       # Resolve - and _ for Namespace usage
       variant_data = {k.replace('-', '_') : v for k, v in self.args.variants[variant_num].items()}
       # Merge and overwrite variant args
-      self.variant_args = argparse.Namespace(**vars(self.args) | variant_data)
+      self.variant_args = argparse.Namespace(**{**vars(self.args), **variant_data})
 
     # Default action is cargo clean, followed by build or user given actions.
     if self.variant_args.cargo: