improve listserv url collection

[Christovis]

This PR contains the following:

• new examples/url_collections/listserv.3GPP.txt file that only contains public mailing lists
• improved routine that really only retrieves mailing list URLs that are public
• a bin/collect_listserv_urls.py that facilitates an easy terminal command to collect all public listserv URLs: $ python bin/collect_urls.py

[codecov-commenter]

Codecov Report

Merging #474 (577c363) into main (a7544e7) will decrease coverage by 0.02%. The diff coverage is 56.66%.

@@            Coverage Diff             @@
##             main     #474      +/-   ##
==========================================
- Coverage   74.24%   74.22%   -0.03%     
==========================================
  Files          16       16              
  Lines        2229     2266      +37     
==========================================
+ Hits         1655     1682      +27     
- Misses        574      584      +10     

Flag	Coverage Δ
unittests	74.22% <56.66%> (-0.03%)	:arrow_down:

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Impacted Files	Coverage Δ
bigbang/listserv.py	79.11% <56.66%> (-1.22%)	:arrow_down:
tests/unit/test_listserv.py	100.00% <0.00%> (ø)

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