Support Python requirements with preceding "v"

[deivid-rodriguez]

We currently raise an error when trying to update requirements like this, for example:

mypy==v1.5.1

Such an error can be reproduced with:

bin/dry-run.rb pip "Rahul2044/cve-bin-tool" --dir="/." --updater-options=grouped_updates_experimental_rules,record_ecosystem_versions,record_update_job_unknown_error,unignore_commands --commit=abaa7571b4aac36489b59137f0ef827c0f4c7ded --cache=files --dep mypy