Some f-string syntax not compatible with python3.7

I just found out on Quentin's shell that pyotb fails to import with python3.7 due to some fstrings in helpers module.
The syntax f"{var=}" was introduced in python3.8.
The error trace was not really easy to understand :

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vidlb/Projets/git/pyotb/pyotb/__init__.py", line 7, in <module>
    from .apps import *
  File "/home/vidlb/Projets/git/pyotb/pyotb/apps.py", line 9, in <module>
    from .helpers import logger, find_otb
  File "<fstring>", line 1
    (prefix=)

I'll remove those problematic strings right now.