Update pipenv_version_resolver.rb (#6837)

Unfortunately, bumping to the latest version of `pipenv` is currently blocked on us dropping support for Python 3.6:
* #6104 (comment)

So for now let's clarify the comment with a TODO... 

This whole bit of code may also get redone if we switch the interface we use:
* #6836

But for now let's capture some notes until we can devote more time to this.

python/lib/dependabot/python/update_checker/pipenv_version_resolver.rb

@@ -239,8 +239,10 @@ def handle_pipenv_errors_resolving_original_reqs(error)
             raise DependencyFileNotResolvable, msg
           end
 
-          # NOTE: Pipenv masks the actualy error, see this issue for updates:
+          # NOTE: Pipenv masks the actual error, see this issue for updates:
           # https://github.com/pypa/pipenv/issues/2791
+          # TODO: This may no longer be reproducible on latest pipenv, see linked issue,
+          # so investigate when we next bump to newer pipenv...
           handle_pipenv_installation_error(error.message) if error.message.match?(PIPENV_INSTALLATION_ERROR_REGEX)
 
           # Raise an unhandled error, as this could be a problem with