Refactor Pypi fetcher class for re-usability

[tdruez]

The previous implementation is too restrictive and returns random distribution.

Changes:

• Add support for no-version PURLs (return latest version)
• Split the code into several methods for versatility
• Add the option to provide a preferred_type such as:

>>> from fetchcode.pypi import Pypi

>>> Pypi.get_download_url("pkg:pypi/requests@2.28.0")
'https://files.pythonhosted.org/packages/e9/23/384d9953bb968731212dc37af87cb75a885dc48e0615bd6a303577c4dc4b/requests-2.28.0.tar.gz'

>>> Pypi.get_download_url("pkg:pypi/requests@2.28.0", preferred_type="bdist_wheel")
'https://files.pythonhosted.org/packages/41/5b/2209eba8133fc081d3ffff02e1f6376e3117e52bb16f674721a83e67e68e/requests-2.28.0-py3-none-any.whl'