Revert "Don't depend on flake8 at runtime (#6830)"

[jeffwidman]

I realized I forgot to update the .python-version file to 3.8, so we're still installing an older flake8 version. And it's also breaking when runs on dependabot-core because it sees this 3.7 and says "that's not a supported version!"

Furthermore, we haven't configured to watch the ./requirements.txt which is separate from ./python/helpers/requirements.txt.

I started to update both of those but the complexity here didn't smell right. This way of installing flake8 is actually different than linters for other native helpers, where we just include them in composer.json, package.json, etc. So the intuitive thing is to install flake8 into ./python/helpers/requirements.txt.

In fact we used to do this, but in
#6830 / f407685 we switched to this new method because flake8 dropped 3.6 support before we were ready to do that here in .

Given that in we're moving toward aligning our support policies with upstream Python... ie, when they EOL a version, we will also drop support for it, then the risk of having a flake8 version that's dropping support for a Python version that isn't yet EOL is quite small.

Interestingly, flake8 was very slow to drop 3.6, but when they did, the maintainer also dropped 3.7 to save himself time even though it was a few months before it was EOL'd. So we could hit this scenario again... but I think the odds of that are somewhat low given the amount of blowback the maintainer got about dropping 3.7 before it was EOL'd. And regardless the consequences are also low... we'd simply revert that flake8 version bump and temporarily hold it back for a few months until the upstream python version was dropped.

Overall this feels like six one way, half a dozen the other as either way we are protecting against something. But switching back to how we do it for our other native helpers is both much simpler setup and also more internally consistent.

So let's solve this by reverting back to the simpler way of installing the linter alongside the helper.

As an aside, restoring this simplicity is yet another win from us choosing to align with upstream Python's EOL policy.

This reverts commit f407685.