README: Replace wget calls with pypi-download

[WilliamDEdwards]

There are two issues that this PR fixes:

• The URL that is retrieved using wget doesn't exist. These URLs were placed in the README 13 years ago, so I think it's likely that PyPI has changed the structure at some point.
• Although it is not literally stated in the README, I take it that pypi-download should be used to download source files from PyPI, instead of wgetting them. According to the changelog, pypi-download was added in version 0.7.1 (2014), so I think it's likely that updating the README was simply forgotten when this tool was introduced.