The dependency resolver in Python's pip command was recently updated in version 20.3. This fundamental change has a number of improvements, but I discovered today a serious drawback of this new machinery. Using the previous resolver, pip allowed you do the following to discover what versions of a package were available:

pip install markdown==

This command provided output like the following:

ERROR: Could not find a version that satisfies the requirement markdown==
(from versions: 1.7, 2.0, 2.0.1, 2.0.2, 2.0.3, 2.1.0, 2.1.1, 2.2.0, 2.2.1,
2.3, 2.3.1, 2.4, 2.4.1, 2.5, 2.5.1, 2.5.2, 2.6, 2.6.1, 2.6.2, 2.6.3, 2.6.4,
2.6.5, 2.6.6, 2.6.7, 2.6.8, 2.6.9, 2.6.10, 2.6.11, 3.0, 3.0.1, 3.1, 3.1.1,
3.2, 3.2.1, 3.2.2, 3.3, 3.3.1, 3.3.2, 3.3.3)
ERROR: No matching distribution found for markdown==

This trick was often useful to discover what new versions (if any) of required packages are available. Sadly, the new machinery no longer produces output like the above. Instead, all you get is this entirely unhelpful message:

ERROR: Could not find a version that satisfies the requirement markdown==
ERROR: No matching distribution found for markdown==

An open bug in the pip project is tracking this issue, but most of the developer responses so far have been of the "we don't have the funding to fix this" variety. There are a number of recommended solutions in the ticket, none of which seem as simple as the previous trick. Hopefully this is something that can be prioritized and fixed soon.