Protontricks fail with python syntax error.

[sizeofbool]

I think latest commit introduced a bug, which makes protontricks fail with python syntax error.

$ python3 -V
Python 3.5.3

$ ./protontricks
  File "./protontricks", line 364
    combo_values = "|".join([f'{app.name}: {app.appid}' for app in steam_apps])
                                                      ^
SyntaxError: invalid syntax

[Matoking]

Python 3.5 doesn't support f-strings (introduced in 3.6).

The following should be compatible with the previous versions.

combo_values = "|".join([
    '{}: {}'.format(app.name, app.appid) for app in steam_apps
])

[sizeofbool]

#41

[sizeofbool]

Python 3.5 doesn't support f-strings (introduced in 3.6).

I see, I thought it was a typo at first glance, now I know the true nature of it.
I've already created pull request with same fix.