…oin's codebase

68faa87 test: use f-strings in mining_*.py tests (fanquake)
c2a5d56 test: use f-strings in interface_*.py tests (fanquake)
86d9582 test: use f-strings in feature_proxy.py (fanquake)
31bdb33 test: use f-strings in feature_segwit.py (fanquake)
b166d54 test: use f-strings in feature_versionbits_warning.py (fanquake)
cf6d66b test: use f-strings in feature_settings.py (fanquake)
6651d77 test: use f-strings in feature_pruning.py (fanquake)
961f581 test: use f-strings in feature_notifications.py (fanquake)
1a546e6 test: use f-strings in feature_minchainwork.py (fanquake)
6679ece test: use f-strings in feature_logging.py (fanquake)
fb63393 test: use f-strings in feature_loadblock.py (fanquake)
e9ca8b2 test: use f-strings in feature_help.py (fanquake)
ff7e330 test: use f-strings in feature_filelock.py (fanquake)
d5a6adc test: use f-strings in feature_fee_estimation.py (fanquake)
a2de33c test: use f-strings in feature_dersig.py (fanquake)
a2502cc test: use f-strings in feature_dbcrash.py (fanquake)
3e2f84e test: use f-strings in feature_csv_activation.py (fanquake)
e2f1fd8 test: use f-strings in feature_config_args.py (fanquake)
36d33d3 test: use f-strings in feature_cltv.py (fanquake)
dca173c test: use f-strings in feature_blocksdir.py (fanquake)
5453e87 test: use f-strings in feature_backwards_compatibility.py (fanquake)
6f3d5ad test: use f-strings in feature_asmap.py (fanquake)

Pull request description:

  Rather than using 3 different ways to build/format strings (sometimes all in the same test, i.e [`feature_config_args.py`](https://github.com/bitcoin/bitcoin/blob/master/test/functional/feature_config_args.py)), consolidate to using [f-strings (3.6+)](https://docs.python.org/3/reference/lexical_analysis.html#f-strings), which are generally more concise / readable, as well as more performant than existing methods.

  This deals with the `feature_*.py`, `interface_*.py` and `mining_*.py` tests.

  See also: [PEP 498](https://www.python.org/dev/peps/pep-0498/)

ACKs for top commit:
  mjdietzx:
    reACK 68faa87
  Zero-1729:
    crACK 68faa87

Tree-SHA512: d4e1a42e07d96d2c552387a46da1534223c4ce408703d7568ad2ef580797dd68d9695b8d19666b567af37f44de6e430e8be5db5d5404ba8fcecf9f5b026a6efb