Add missing dependency

Without that, starting from a debian « naked » system,  it will fail at the `pip install wheel` stage.

docs/installation/debian.rst

@@ -23,7 +23,7 @@ default on system. You can install them using:
 .. code-block:: shell
 
     sudo apt-get update
-    sudo apt-get install curl python3-venv git unzip
+    sudo apt-get install curl python3-pip python3-venv git unzip
 
 
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