Find a way to allow prereleases, but on an optin basis.

[hartym]

Easy to do the following, but one should opt-in to prereleases.

--- a/medikit/feature/python.py
+++ b/medikit/feature/python.py
@@ -309,7 +309,7 @@ class PythonFeature(Feature):
                     tmpfile.name, finder=repository.finder, session=repository.session, options=pip_options
                 )
             )
-            resolver = Resolver(constraints, repository, prereleases=False, clear_caches=False, allow_unsafe=False)
+            resolver = Resolver(constraints, repository, prereleases=True, clear_caches=False, allow_unsafe=False)

             self.render_file_inline(
                 'requirements{}.txt'.format('-' + extra if extra else ''),