Currently an app's Python version can be set via a few different means:
- explicitly by the user (via `runtime.txt` or `Pipfile.lock`)
- implicitly via the sticky versions feature (for existing apps)
- implicitly via default version for new apps / those with empty cache
In order to determine the priority of features like automatic Python
patch version upgrades for sticky-versioned apps, it's useful to have
metrics for these.
There were previously no tests for either the sticky versions feature,
or changing the Python version by updating the `runtime.txt` file, so
I've added some now (given that I updated the conditional to add the
metrics, so useful to have coverage).
I've also removed the confusing overwrite of `DEFAULT_PYTHON_VERSION`
with the cached version, and kept them as two separate variables.
Closes @W-8099632@.
Closes @W-8099645@.
Ed Morley
eb6ee49dfe