97c738ff1b [tests] Recommend f-strings for formatting, update feature_block to use them (Anthony Towns)
8ae9d314e9 Bump minimum python version to 3.6 (Anthony Towns)
Pull request description:
Python 3.5 has reached [end-of-life](https://devguide.python.org/#status-of-python-branches) as of September 2020, and 3.6 has some moderately nice [features](https://docs.python.org/3/whatsnew/3.6.html):
- `f'x = {x}'` as an alternative to `'x = {}'.format(x)` format strings (cf https://github.com/bitcoin/bitcoin/pull/13718#issuecomment-406591027)
- underscore separators for large numbers, like `1_234_567`
- improvements to async
- improvements to typing module
Note that 3.6 is not available in xenial (16.04), but is available in bionic (18.04), while focal (20.04) has 3.8. CentOS 7 and 8 have 3.6.8, Debian stable has 3.7.3, and [gentoo and arch already had 3.6 and 3.7 in 2018](https://github.com/bitcoin/bitcoin/pull/14954#issuecomment-447118707).
ACKs for top commit:
MarcoFalke:
re-ACK 97c738ff1b
Tree-SHA512: ec7fce68845edde4d61a42de12c065fd49e5217311a6fda1323206f091a0afd50f293645dffc27d420127e4e5deb864e953f1b67eff735a0dfbbedd7899a9d60
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