How to create a deb package for a python3 script?

Creating a .deb for a python3 script is very simple, and only requires a few changes in debian/rules and debian/control if you're familiar with python2 packaging.

In a nutshell:

1. Create the package source dir

   mkdir myscript-0.1

2. Copy your python3 script (or the sample script below) to the source dir

   cp ~/myscript myscript-0.1
   cd myscript-0.1

   Sample script:

   #!/usr/bin/python3
   
   if __name__ == '__main__':
       print("Hello world")

3. Create the packaging skeleton (debian/*)

   dh_make -s --indep --createorig

4. Remove the example files

   rm debian/*.ex debian/*.EX debian/README.*

5. Edit debian/control

   Replace its content with the following text:

   Source: myscript
   Section: utils
   Priority: optional
   Maintainer: Name, 
   Build-Depends: debhelper (>= 9), python3
   Standards-Version: 3.9.5
   X-Python3-Version: >= 3.2
   
   Package: myscript
   Architecture: all
   Depends: ${misc:Depends}, ${python3:Depends}
   Description: insert up to 60 chars description
    insert long description, indented with spaces
   

6. debian/install must contain the script to install as well as the target directory

   echo myscript usr/bin > debian/install

7. Edit debian/rules

   Replace its content with the following text:

   #!/usr/bin/make -f
   
   %:
       dh $@ --with=python3

   Note: it's a TAB before dh $@, not four spaces!

8. Build the package

   debuild -us -uc

You will get a few Lintian warnings/errors but your package is ready to be used:

../myscript_0.1-1_all.deb