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

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.

.github/workflows/ci.yml

@@ -241,9 +241,4 @@ jobs:
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true
-      - uses: actions/setup-python@v4
-        with:
-          python-version-file: .python-version
-          cache: pip
-      - run: pip install -r requirements.txt
       - run: ./bin/lint

bin/lint

@@ -15,5 +15,3 @@ shellcheck \
   "$@"
 
 bundle exec rubocop
-
-flake8 python/helpers/. --count --exclude=./.*,./python/spec/fixtures --show-source --statistics

python/helpers/requirements.txt

@@ -1,5 +1,6 @@
 pip==23.2.0
 pip-tools==7.2.0
+flake8==5.0.4
 hashin==0.17.0
 pipenv==2022.4.8
 pipfile==0.0.2

python/script/ci-test

@@ -2,5 +2,7 @@
 
 set -e
 
+pyenv exec flake8 helpers/. --count --exclude=./.*,./python/spec/fixtures --show-source --statistics
+
 bundle install
 bundle exec parallel_test spec/ -n "$CI_NODE_TOTAL" --only-group "$CI_NODE_INDEX" --group-by filesize --type rspec --verbose