Returns content_versions (each with date, amendment_date, issue_date,
identifier, name, substantive and removed), every version eCFR lists:
all of eCFR's pages are read, so Part 52's 2,264 versions are all
there. Versions are grouped by section, oldest first.

since_date/until_date (YYYY-MM-DD) keep versions dated in that range.
per_page (default 200, max 1000) and page step through long histories;
total_count and total_pages say how many there are.

'substantive' is eCFR's flag: true when the version's text differs from
the one before, including editorial edits such as citation fixes and
the "Link to an amendment published at ..." notice eCFR adds when a
rule is published; false when re-issued with no text change. To see
what actually changed, use compare_versions on the dates.

'removed' true means the section was removed on that date.

Version metadata and text snapshots may include dates before 2017.
Availability varies by title and section. Use the returned history
dates for reads or comparisons; a missing old snapshot produces an
availability error. A 2017-01-01 baseline often means unchanged since
before 2017.

part/subpart/section accept int or string.