name_segwit.py functional test uses post-Python 3.5 syntax

[yanmaani]

name_segwit.py doesn't run on Python 3.5. All functional tests are supposed to run on Python 3.5: test/functional/README.md: The oldest supported Python version is specified in doc/dependencies.md.

doc/dependencies.md: These are the dependencies currently used by Bitcoin Core. You can find instructions for installing them in the build-*.md file for your platform.

Dependency	Version used	Minimum required	CVEs	Shared	Bundled Qt library
Python (tests)		3.5

The offending line is line 41 of name_segwit.py:

      f"-segwitheight={SEGWIT_ACTIVATION_HEIGHT}"

Useful skills:

Rudimentary Python experience

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[JeremyRand]

Is this still a thing given https://github.com/namecoin/namecoin-core/commit/8ae9d314e9af7bcce1e8bc52f0317b9d565109bf ?