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:

charge-lnd/charge_lnd/charge_lnd.py

Line 102 in e3658d2

s = ' ➜ ' + fmt.col_hi(new_base_fee_msat)

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 :)