Issue with pypi decorator and special characters in package name

[Sakari Ikonen]

The current PyPI implementation breaks if a package name or version has special characters in it that require URL encoding due to this leading to a mismatch in the cache path lookup.

Easy reproduction:
publish your favourite Python package to a private repository with a custom version, e.g. 1.0.0.1+devtest and try to install this with the decorator.