* guix/import/pypi.scm (maybe-inputs): Add INPUT-TYPE argument, and use it.
(test-section?): New predicate.
(parse-requires.txt): Collect the optional test inputs, and return them as the
second element of the returned list.
(parse-wheel-metadata): Likewise.
(guess-requirements): Adapt.
(make-pypi-sexp): Likewise, and include the test inputs requirements as native
inputs in the returned package expression.

* tests/pypi.scm (test-requires.txt): Include a test section in the
test-requires.txt data.
(test-requires.txt-beaker): New variable.
("parse-requires.txt"): Adapt.
("parse-requires.txt - Beaker"): New test.
("parse-wheel-metadata, with extras"): Adapt.
("parse-wheel-metadata, with extras - Jedi"): Adapt.
("pypi->guix-package, no wheel"): Re-indent, and add the expected
native-inputs.
("pypi->guix-package, wheels"): Likewise.