Bring back filename parameter to compression_wrapper

[piskvorky]

Smart_open 1.8.4 used to have a nifty _compression_wrapper function. It was internal (_ prefix) but generally useful.

That function doesn't exist any more in smart_open 2.0.0. It was promoted to a public compression.compression_wrapper which is almost identical, but not quite. The new function doesn't accept a filename, and is instead hard-wired to extract the filename from file_obj.name.

This is undesirable in our system, where hacking file_obj to add the .name attribute is complicated (and unnecessary).

This PR brings back the option of supplying own filename. It's fully backward compatible: with no filename specified, it will fall back to file_obj.name like before.

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[piskvorky]

Also had to fix some unrelated code because CI was failing flake8 with E741 ambiguous variable name 'l'.

[mpenkov]

Looks good, minor nitpicks.

[mpenkov]

Do we want this in the change log? It seems relatively minor, so we can leave it out, in my opinion.

[piskvorky]

Definitely, very minor (a user base of one, most likely). Thanks for the review!