recommonmark doesn't build because of failing tests with sphinx 3

[DieGoldeneEnte]

Describe the bug
recommonmark doesn't build because of failing tests with sphinx 3.
This is also noted here.
This only affects the python3 variants since python2 uses a different (older) version.

To Reproduce
Steps to reproduce the behavior:

1. build recommonmark on master

Expected behavior
The package builds.

Notify maintainers

@FRidh

some output

WARNING: while setting up extension sphinx.domains.changeset: directive 'versionadded' is already registered, it will be overridden
WARNING: while setting up extension sphinx.domains.changeset: directive 'versionchanged' is already registered, it will be overridden
WARNING: while setting up extension sphinx.domains.index: directive 'index' is already registered, it will be overridden
WARNING: while setting up extension sphinx.domains.index: role 'index' is already registered, it will be overridden
WARNING: while setting up extension sphinx.domains.math: role 'eq' is already registered, it will be overridden
WARNING: html_static_path entry '_static' does not exist
=============================== warnings summary ===============================
tests/test_basic.py:35
  /build/source/tests/test_basic.py:35: DeprecationWarning: invalid escape sequence \
    """

tests/test_basic.py:52
  /build/source/tests/test_basic.py:52: DeprecationWarning: invalid escape sequence \
    """

tests/test_basic.py: 17 tests with warnings
  /build/source/recommonmark/parser.py:75: UserWarning: Container node skipped: type=document
    warn("Container node skipped: type={0}".format(mdnode.t))

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=========================== short test summary info ============================
FAILED tests/test_sphinx.py::GenericTests::test_code - AssertionError: '<pre>...
FAILED tests/test_sphinx.py::GenericTests::test_headings - AssertionError: '<...
FAILED tests/test_sphinx.py::GenericTests::test_image - AssertionError: '<p><...
FAILED tests/test_sphinx.py::GenericTests::test_links - AssertionError: 'This...
=========== 4 failed, 19 passed, 5 deselected, 19 warnings in 1.51s ============
builder for '/nix/store/4bc7k7b036gzam1g8v3i61ip7vjnbk4y-python3.8-recommonmark-0.6.0.drv' failed with exit code 1
error: build of '/nix/store/4bc7k7b036gzam1g8v3i61ip7vjnbk4y-python3.8-recommonmark-0.6.0.drv' failed

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

 - system: `"x86_64-linux"`
 - host os: `Linux 5.7.6, NixOS, 20.09pre234194.8d05772134f (Nightingale)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.7`
 - channels(holger): `""`
 - channels(root): `"nixos-20.09pre234194.8d05772134f"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
- python37Packages.recommonmark
- python38Packages.recommonmark
# a list of nixos modules affected by the problem
module:

[DieGoldeneEnte]

Just tested reverting sphinx back to 2.3.1 (I reverted to 84cf00f) and it works again. Maybe we need a sphinx2 package?

Edit: fixed the commit hash