[packaging][good first issue] Set build backend in `pyproject.toml`

[webknjaz]

Apparently, #986 was reported a while ago but the commit claiming to fix it (7b3f3ef) is a hack, not a correct fix. At some point, setuptools' docs have indeed suggested listing wheel as a direct dependency of the build environment but that was a mistake that I've fixed long ago: setuptools used to inject the wheel dep only when building wheels (not just sdist) since the inception of the implementation using one of the PEP 517 hooks for additional deps, but about two years ago the respective portions of wheel moved into setuptools and that dep isn't even used.

Action items:

• Remove wheel from the [build-system].requires @ pyproject.toml
• Add build-backend = "setuptools.build_meta" into the [build-system] section — this is what pygit2 1.1.1 missing build requirements in pyproject.toml #986 really needed

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The reason #986 happened was that at the time, when there was no build-backend set, a default of setuptools.build_meta:__legacy__ was assumed and pip install would not use any build isolation, nor would it get the hook-provided deps. In that mode, building a wheel (as a part of pip's installation process) was optional and if wheel was present, it'd make+cache a wheel and unzip it into the respective site-packages/, otherwise, it'd fall back to the ancient scary setup.py install which would be doing all sorts of nasty things, hence the warning.