The LEPL dependency was triggering a lot of deprecation warnings of the
form:

    venv/lib/python3.5/site-packages/lepl/matchers/support.py:497:
    DeprecationWarning: inspect.getargspec() is deprecated, use
    inspect.signature() instead
    argspec = getargspec(func)

It turns out that LEPL was only used by the rfc6266_parser package, which
itself was only used in one place to generate utf8-compliant
Content-Disposition headers.

This issue was noticed here:
https://github.com/SWW13/python-rfc6266-parser/issues/2
Unfortunately it is quite difficult to extract LEPL from the
rfc6266-parser package.

The rfc6266-parser package (https://pypi.org/project/rfc6266-parser/) is
itself a fork of the now-unmaintained rfc6266 package
(https://pypi.org/project/rfc6266/). Thus, it became high time to get
rid of this package. The FileResponse object can appropriately set the
Content-Disposition header, and thus replace the rfc6266 functionality,
since Django 2.0: https://code.djangoproject.com/ticket/16470

In our testing, the FileResponse object correctly set the
`filename*=utf-8''` value, following the RFC. The only difference is
that it does not provide "filename" fallback value, as expressed in the
RFC: https://tools.ietf.org/html/rfc6266#appendix-D

With rfc6266_parser:

    >> import rfc6266_parser
    >> rfc6266_parser.build_header("my_file_é.csv", filename_compat="video_urls.csv")
    b"attachment; filename=video_urls.csv; filename*=utf-8''my_file_%C3%A9.csv"

With FileResponse we have:

    >> from django.http import FileResponse
    >> import io
    >> response = FileResponse(io.StringIO(), as_attachment=True, filename="my_file_é.csv", content_type="text/csv")
    >> response.get("Content-Disposition")
    "attachment; filename*=utf-8''my_file_%C3%A9.csv"

We consider that this is a sufficiently minor difference, that will
impact very few browsers.

We introduce a new documentation target, where we use the featuretoggles
Sphinx extension from code-annotations to generate human-readable
documentation of feature toggles in edx-platform. The annotation report
is generated on-the-fly based on the standard feature toggle
configuration file in code-annotations.

In addition, we add new doc.in & doc.txt requirement files that will be
pip-installed by readthedocs to generate the documentation targets.

- Originally pinned in PR #23566
- pytest was unpinned in PR #24717 and upgraded to v6+