* guix/import/pypi.scm: Export PARSE-REQUIRES.TXT.
(clean-requirement): Move procedure to the top level.
(guess-requirements): Move the READ-REQUIREMENTS procedure to the top level,
and rename it to PARSE-REQUIRES.TXT. Move the CLEAN-REQUIREMENT procedure to
the top level. Move the COMMENT? functions inside the PARSE-REQUIRES.TXT
procedure.
(parse-requires.txt): Add a SECTION-HEADER? predicate, and use it to prevent
parsing optional requirements.
* tests/pypi.scm (test-requires-with-sections): New variable.
("parse-requires.txt, with sections"): New test.
c4797121be