Pipenv support for python versions < 3.6

[R-Faraji]

As for now, using pipenv to install packages and their dependencies does not work when using a virtual env with a python version < 3.6.
Which is a significant problem for managing dependencies, since relying only on pip is not a good thing.
The main error from python version < 3.6 is for example with python version 3.5.1 :

Locking Failed! 

  File "/home/richard/.local/pipx/venvs/pipenv/lib/python3.8/site-packages/pipenv/resolver.py", line 34
    path_options = [name, f"{name}.py"]
                                     ^
SyntaxError: invalid syntax

So not only is it gimping the project, but if the locking part of pipenv is not working might no as well not use pipenv since all of the caviar is that the locking file prevents problems from the dependencies.