Bug 1638783 - Install Mercurial with pip3 rather than pip, which no longer exists on new Ubuntu/Debian versions r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D75816

python/mozboot/mozboot/debian.py

@@ -209,4 +209,4 @@ class DebianBootstrapper(
 
         # pip.
         assert res == 1
-        self.run_as_root(['pip', 'install', '--upgrade', 'Mercurial'])
+        self.run_as_root(['pip3', 'install', '--upgrade', 'Mercurial'])