Explicitly use python version for poller-wrapper.py

This is a followup to OBS-1302 Running poller-wrapper.py on CentOS 8 results in the following error:

/usr/bin/env: ‘python’: No such file or directory

CentOS 8 no longer honors shebang lines written as #!/usr/bin/env python

As a workaround users can specify a default python version by running the command

alternatives --set python /usr/bin/python3
alternatives --set python /usr/bin/python2{{}}

According to PEP 0394 "Some Linux distributions will not provide a {{python command at all by default, but will provide a python3 command by default."}}

{{}}