fetch: add pypi backend

[stdmnpkg]

find source code url for pypi package is not trivial, so i automated it.

[what-the-diff]

PR Summary

• Improved Download Handling:
  • The tarball_fetch function which is used to download information has been enhanced. The direct calls to a command-line tool wget were replaced with a developed wget_download helper function that simplifies the download process. We also added error handling measures for when downloads fail, ensuring the system can react accordingly.
• New Downloading Function:
  • A new function called wget_download was added. It's a more organized way to handle downloads using the wget command-line tool, preventing the overwriting of already downloaded files.
• Development of PyPI Package Fetching:
  • There's a new pypi_fetch function that fetches packages from PyPI, the Python Package Index, through the use of PyPI's API. It obtains the source URL of a package and identifies how the package has been compressed.
• Expanded Object Handling:
  • The source processing methods, grouped together in the processes mapping, have been expanded to support 'PYPI' as a source, connecting to the newly created pypi_fetch function.
• Enhanced Function Parsing:
  • An update was made to the parse_fetch_options function that allows it to parse for 'version' and set it to a specific value. This is an addition to the pre-existing ability to parse for 'commit'.

[stdmnpkg]

usage:

SRCS="pypi::version=$VER::$PKGNAME"

for example:

SRCS="pypi::version=3.0.3::flask"