Bug#951592: sagemath-common: SyntaxWarnings setting up sagemath-common

[John Scott]

Control: affects -1 python3-sagetex

Upgrading to 9.0-3 and installing SageTeX says
Setting up python3-sagetex (3.4+ds-1) ...
/usr/lib/python3/dist-packages/sagetexparse.py:135: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if t.format is not '':
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