Allow passing --no-upgrade option

[ssbarnea]

This makes this option similar with the other boolean ones and allows
calling the tools with explicit --no-upgrade.

This change ease integration with other tools like CI/CD pipelines
where user may want to change the default behavior related to
upgrades.

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• Provided the tests for the changes.
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[atugushev]

LGTM 👍 Maybe "enhancement"?

[ssbarnea]

LGTM 👍 Maybe "enhancement"?

I see it a as a bugfix, and I do not want to wait until 6.3.0 is ready, especially as lack of it prevents use with tox posargs where I want to do something like

command = pip-compile {posargs:--upgrade} ....

This construct allows running that tox env with only compilation and still allow user to call it with tox -e envname -- --no-upgrade to disable the implicit upgrade (something very useful for CI pipeline, where we only want to check if compile deps are valid, not to upgrade all deps).