Stop manually installing python

[jeffwidman]

Depends on:

• Drop python 3.6 #7610

In the python dockerfile, we pre-install python 3.7-3.11, then tar it
to save space.

Now that we're not supporting python <= 3.6, we no longer have any
reason to run pyenv install, so we can dop that and simplify our code.

[deivid-rodriguez]

Seems straightforward but I did have a question!

[deivid-rodriguez]

What happens if we find a python version higher than the latest that we support, for example, right after a new Python release.

[jeffwidman]

Great question!

The behavior should remain the same as today:

1. Newer patch releases are ignored and we use the matching major.minor pre-installed, until someone bumps the patch release we use (more details in Stop explicitly specifying python patch versions #7615).
2. Newer minor versions (eg 3.12 that is getting released in a few months) will throw an error about an unsupported Python version.

This is a reasonable case where we'd want to continually to manually install, but OTOH the time/network cost of doing that dynamically on every job that needs it is quite expensive.

So far, it's worked out okay that whenever a new minor release happens, someone from the community pretty quickly puts up an upgrade PR.

Ideally we actually start scheduling this into our ecosystem calendar so that we're ready with a PR the day it gets released. But we're lacking enough maintainer time to stay on top of our deprecations, let alone new versions, so might be a bit til we're that organized.

[jeffwidman]

Also, all the logic for choosing the python version happens before we even get to this code... it's all happening over in:

dependabot-core/python/lib/dependabot/python/language_version_manager.rb

Lines 48 to 74 in bfb40cc

	# Ideally, the requirement is satisfied by a Python version we support
	requirement =
	Python::Requirement.requirements_array(requirement_string).first
	version =
	PythonVersions::SUPPORTED_VERSIONS_TO_ITERATE.
	find { |v| requirement.satisfied_by?(Python::Version.new(v)) }
	return version if version
	# If not, and we're dealing with a simple version string
	# and changing the patch version would fix things, we do that
	# as the patch version is unlikely to affect resolution
	if requirement_string.start_with?(/\d/)
	requirement =
	Python::Requirement.new(requirement_string.gsub(/\.\d+$/, ".*"))
	version =
	PythonVersions::SUPPORTED_VERSIONS_TO_ITERATE.
	find { |v| requirement.satisfied_by?(Python::Version.new(v)) }
	return version if version
	end
	# Otherwise we have to raise, giving details of the Python versions
	# that Dependabot supports
	msg = "Dependabot detected the following Python requirement " \
	"for your project: '#{requirement_string}'.\n\nCurrently, the " \
	"following Python versions are supported in Dependabot: " \
	"#{PythonVersions::SUPPORTED_VERSIONS.join(', ')}."
	raise DependencyFileNotResolvable, msg