Handle complaints about shebangs during RPM build

When building the Dangerzone RPMs, we were seeing the following shebang
warnings:

    + /usr/lib/rpm/redhat/brp-mangle-shebangs
    mangling shebang in /usr/lib/python3.12/site-packages/dangerzone/conversion/doc_to_pixels.py from /usr/bin/env python3 to #!/usr/bin/python3
    mangling shebang in /usr/lib/python3.12/site-packages/dangerzone/conversion/common.py from /usr/bin/env python3 to #!/usr/bin/python3
    mangling shebang in /usr/lib/python3.12/site-packages/dangerzone/conversion/pixels_to_pdf.py from /usr/bin/env python3 to #!/usr/bin/python3
    mangling shebang in /etc/qubes-rpc/dz.ConvertDev from /usr/bin/env python3 to #!/usr/bin/python3
    mangling shebang in /etc/qubes-rpc/dz.Convert from /bin/sh to #!/usr/bin/sh

These warnings are benign in nature, but coupled with #727, they could
lead to incorrect file permissions.

Remove shebangs from the following files, since they are not executed
directly, but are imported instead:

    dangerzone/conversion/common.py
    dangerzone/conversion/doc_to_pixels.py
    dangerzone/conversion/pixels_to_pdf.py

Also, accept the suggestions by Fedora (/bin/sh -> /usr/bin/sh,
/usr/bin/env python3 -> /usr/bin/python3) for the following files:

    qubes/dz.Convert
    qubes/dz.ConvertDev

Refs #727

dangerzone/conversion/common.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 import asyncio
 import glob
 import json

dangerzone/conversion/doc_to_pixels.py

@@ -1,4 +1,3 @@
-#!/usr/bin/env python3
 import asyncio
 import glob
 import os

dangerzone/conversion/pixels_to_pdf.py

@@ -1,4 +1,3 @@
-#!/usr/bin/env python3
 """
 Here are the steps, with progress bar percentages:
 

qubes/dz.Convert

@@ -1,2 +1,2 @@
-#!/bin/sh
+#!/usr/bin/sh
 python -m dangerzone.conversion.doc_to_pixels

qubes/dz.ConvertDev

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/python3
 
 import asyncio
 import glob