PyPI does not list which packages depend on yours. Use a local tool if the package is installed, or a third-party index. On PyDeps, open the Dependents tab (or append #dependents) to see reverse dependents alongside the forward tree, CVEs, licenses, and offline bundles.
Python packaging allows dynamic dependencies. Building an accurate reverse graph requires scanning essentially every package on the index. See warehouse issue 18196.
In an installed environment: pipdeptree --reverse --packages urllib3. This only answers what in the current venv requires the package.
Wheelodex, libraries.io, deps.dev, and PyDeps each expose reverse dependents from published metadata. Results are approximate because of dynamic metadata and environment markers.