Python 2 interpreter instead of 3 in Geany

On my PC, Python 2 and 3 are installed – a query of the versions, using the terminal, outputs the following:

$ python2 --version 
Python 2.7.14

$ python3 --version
Python 3.6.3

I would like to learn Python 3, and I use Geany under Xubuntu 17.10 as my development environment. The shebang line reads as follows:

#!/usr/bin/env python3

I do a query for the python version in my little program, the output tells me that Python 2 interpreter is used:

print(sys.version_info)
sys.version_info(major=2, minor=7, micro=14, releaselevel='final', serial=0)

How can that be, as I specify Python 3 in the Shebang? I also wrote the path to the Python 3 interpreter there, but that did not help.

So, here’s my concrete question: How can I achieve that my program gets interpreted by Python 3, and not 2? I could not find an answer in the WWW – I seem to be the only person having this problem. Deinstalling Python 2 is no option, as many applications need this version 2. For any hints where I could start; I’d be very thankful.