Update README: python-dev is no more

Author: oremanj

README.rst

@@ -79,7 +79,7 @@ Before installing, ensure you have:
 
 On Debian or Ubuntu, install these files with::
 
-    apt-get install build-essential python-dev libnetfilter-queue-dev
+    apt-get install build-essential python3-dev libnetfilter-queue-dev
 
 From PyPI
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