LISTSERV 16.5 url lists to /examples/url_collections/

[Christovis]

This PR addresses #419 and will implement the following:

• add /examples/url_collections/listserv.3GPP.txt
• add method to /bigbang/listserv.py to generate the above
• new handling of corner cases (e.g. missing date) in ListservMessage.format_header_content()
• add better docstrings to /bigbang/listserv.py in some places
• adjust tests/webscraping/test_listserv.py for renaming of some variables and include larger number of mailing list to make sure that corner cases named above are included and tested for

[sbenthall]

Hello. Thanks for this PR. Sorry for the slow response -- I've been up against a major major deadline this week.

I'm getting some failing tests. I wonder what's up.

https://gist.github.com/sbenthall/8e0110c01e821fa127bc0c20bf3007af

[Christovis]

Tststs... sorry about that. The latest commit should fix the issue. The issues was: 1) I overlooked in a @classmethod to address a class object properly: cls.empty_header 2) Didn't take proper care of a corner case in which an Email header does not contain a date

[sbenthall]

Still getting test failures.

Looks like something about connections being tried, and retried, and retried, then aborted.

Is this pinging an external server? https://gist.github.com/sbenthall/29486b79ca7ea014f07e4f59e69391ec

[Christovis]

Yes, the webscraping tests require a good internet connection and can take much longer than the tests in /tests/unit/ (take ~5min for me). It is very probable that the error is caused by a weak connection. Maybe Nick can try as it worked for him the last time (it is working for me).

[sbenthall]

Ok it passed for me this time!