retry fetching extensions from PyPI with alternative download_filename

[smoors]

retry by replacing hyphen with underscore in the name: abc-def-%(version).tar.gz -> abc_def-%(version).tar.gz

only done if:

• it's an extension
• download from PyPI
• there's only 1 source url
• default filename is used (no sources or source_tmpl specified)

at some point we should probably switch to using the underscore in the first try, as that is the recommended name format, see
https://packaging.python.org/en/latest/specifications/source-distribution-format/#source-distribution-file-name

recommendation: always specify the name of the package as shown on PyPI:

• it ensures EB tries to download the 2 name formats if there are hyphens in the name
• users are more likely to search for it
• it standardizes the naming of extensions in Lmod (module spider), avoiding that the same package is available with 2 different names

[Flamefire]

I'd suggest to rename warning_only to something like is_pypi_source and change this to:

Suggested change

	if not src_path:
	if not src_path and is_pypi_source:

That makes it clear when/why this is retried as the error if the file isn't found and we don't reach this code unless is_pypi_source == True is a bit hidden by src_path = ... (indicating it returns) especially with the (redundant?) check for src_path below

[smoors]

the check for src_path is indeed redundant, but i figured it might be good to keep it as a safety net for future changes to obtain_file, i don't know. happy to remove it if you prefer.

[smoors]

done in 04da499

[Flamefire]

I would remove it as it may lead to confusion here as it basically communicates that None (or empty) would be valid results of obtain_file which they aren't. Doesn't need to be done bow/in this PR though, I've just fallen into this "trap" during the review of this PR and had to check the code of obtain_file to verify.

[Flamefire]

at some point we should probably switch to using the underscore in the first try,

Couldn't there be packages like very_cool-software? And some packages are named foo_bar or on PyPI so we should keep the spelling of PyPI although in general it should by hyphens.

[smoors]

at some point we should probably switch to using the underscore in the first try,

Couldn't there be packages like very_cool-software? And some packages are named foo_bar or on PyPI so we should keep the spelling of PyPI although in general it should by hyphens.

yes, we should keep the spelling of PyPI. i meant first trying with underscore for the tarball, as the hyphen should normally only be used to separate the name from the version.