chore: modernize f-string usage

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 41.93548% with 18 lines in your changes missing coverage. Please review.

Project coverage is 56.98%. Comparing base (8675745) to head (42241b3). Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
cashu/lightning/blink.py	0.00%	5 Missing :warning:
cashu/core/crypto/secp.py	0.00%	3 Missing :warning:
cashu/wallet/cli/cli.py	40.00%	3 Missing :warning:
cashu/lightning/lndrest.py	0.00%	1 Missing :warning:
cashu/mint/verification.py	0.00%	1 Missing :warning:
cashu/tor/tor.py	0.00%	1 Missing :warning:
cashu/wallet/cli/cli_helpers.py	0.00%	1 Missing :warning:
cashu/wallet/nostr.py	0.00%	1 Missing :warning:
cashu/wallet/v1_api.py	66.66%	1 Missing :warning:
cashu/wallet/wallet_deprecated.py	0.00%	1 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (8675745) and HEAD (42241b3). Click for more details.

HEAD has 7 uploads less than BASE

| Flag | BASE (8675745) | HEAD (42241b3) | |------|------|------| ||9|2|

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #627 +/- ## =========================================== - Coverage 69.02% 56.98% -12.04% =========================================== Files 92 98 +6 Lines 8674 10564 +1890 =========================================== + Hits 5987 6020 +33 - Misses 2687 4544 +1857 ```

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[callebtc]

@prusnak thanks! it's hard to see what changed with the force pushes. I like this PR in almost all places but some strings are definitely harder to read as f-strings. Did you consider the comments above or are they "resolved" because of the force-push?

[prusnak]

Did you consider the comments above or are they "resolved" because of the force-push?

Yes, I addressed both comments.