Instrument Python version metric (dependabot#7617)

We want to know what versions of python are required by our users, so
this is an initial stab at collecting some data.

Ideally we capture the minimum version, the maximum version, and the raw
data from each manifest file that allowed us to calculate the min/max...
unfortunately this isn't so clean/polished, but we need to start
somewhere and even a primitive form of this metric will provide enough
information for us to understand the user impact of dropping python 3.6.

---------

Co-authored-by: Tom Christensen <pavera@github.com>

python/lib/dependabot/python/file_fetcher.rb

@@ -37,6 +37,28 @@ def self.required_files_message
           "or a Pipfile."
       end
 
+      def ecosystem_versions
+        # Hmm... it's weird that this calls file parser methods, but here we are in the file fetcher... for all
+        # ecosystems our goal is to extract the user specified versions, so we'll need to do file parsing... so should
+        # we move this `ecosystem_versions` metrics method to run in the file parser for all ecosystems? Downside is if
+        # file parsing blows up, this metric isn't emitted, but reality is we have to parse anyway... as we want to know
+        # the user-specified range of versions, not the version Dependabot chose to run.
+        python_requirement_parser = FileParser::PythonRequirementParser.new(dependency_files: files)
+        language_version_manager = LanguageVersionManager.new(python_requirement_parser: python_requirement_parser)
+        {
+          languages: {
+            python: {
+              # TODO: alternatively this could use `python_requirement_parser.user_specified_requirements` which
+              # returns an array... which we could flip to return a hash of manifest name => version
+              # string and then check for min/max versions... today it simply defaults to
+              # array.first which seems rather arbitrary.
+              "raw" => language_version_manager.user_specified_python_version || "unknown",
+              "max" => language_version_manager.python_major_minor || "unknown"
+            }
+          }
+        }
+      end
+
       private
 
       def fetch_files

python/spec/dependabot/python/file_fetcher_spec.rb

@@ -507,6 +507,12 @@
         expect(file_fetcher_instance.files.count).to eq(2)
         expect(file_fetcher_instance.files.map(&:name)).to include("setup.cfg")
       end
+
+      it "exposes the expected ecosystem_versions metric" do
+        expect(file_fetcher_instance.ecosystem_versions).to eq({
+          languages: { python: { "max" => "3.11", "raw" => "unknown" } }
+        })
+      end
     end
 
     context "with a requirements.txt, a setup.py and a requirements folder" do