Из документации: upquote=⟨true|false⟩ false determines whether the left and right quote are printed ‘’ or `'. This key requires the textcomp package if true.

\documentclass{article}
\usepackage{listings}
\usepackage{textcomp}

\begin{document}
\begin{lstlisting}[language=Python,upquote=true]
print('Hello world!')
\end{lstlisting}