Default Julia version for the Project.toml buildpack is the latest, but the logic indicates it was supposed to be 1.6.x

[consideRatio]

It turns out tha the default Julia version for the Project.toml buildpack is the latest version (currently 1.8.2), but the logic indicates it was supposed to be 1.6.x. This caused quite a bit of confusion for me in #1188.

repo2docker/repo2docker/buildpacks/julia/julia_project.py

Lines 40 to 58 in c981b3b

	@property
	def julia_version(self):
	if os.path.exists(self.binder_path("JuliaProject.toml")):
	project_toml = toml.load(self.binder_path("JuliaProject.toml"))
	else:
	project_toml = toml.load(self.binder_path("Project.toml"))
	try:
	# For Project.toml files, install the latest julia version that
	# satisfies the given semver.
	compat = project_toml["compat"]["julia"]
	except:
	# Default version which needs to be manually updated with new major.minor releases
	compat = "1.6"
	match = find_semver_match(compat, self.all_julias)
	if match is None:
	raise RuntimeError("Failed to find a matching Julia version: {compat}")
	return match

There are some action points here.

1. Define the desired behavior
2. Add unit tests for the logic that provides us with a julia version using a value from a Project.toml's compat -> julia key
3. Update the julia_project.py buildpack making the logic easier to read