make-autosuspend-rules: restore compatibility with Python3 < 3.6

make-autosuspend-rules: restore compatibility with Python3 < 3.6

The f'...' format was introduced in Python 3.6 ( https://www.python.org/dev/peps/pep-0498/ )
and returns an error when systemd is built on a system with an older Python3 version:

<...>
  File /home/bluca/git/systemd/tools/make-autosuspend-rules.py, line 15
    print(f'pci:v{vendor:08X}d{device:08X}*')
                                           ^
SyntaxError: invalid syntax
[2/388] Generating version.h with a custom command.
ninja: build stopped: subcommand failed.
$ python3 --version
Python 3.5.6

Use an older format to keep backward compatibility.