Migrate to f-string

[roshii]

Interpolate values into strings using f-strings (allows to cope with integer type variables, preventing errors)

For context: I implemented this fix following errors while running program. I do not have logs any longer but I remember it was failing to interpolate int at the following line:

https://github.com/accumulator/charge-lnd/blob/e3658d2295cb12f37ce61d870402e14241671a59/charge_lnd/charge_lnd.py#L102

I'm now reading your fmt module and thinking fix should have been implemented there... Anyhow, fstrings worked just fine for me, charge-lnd works again :)