Build currently fails with issue around `SymbiFlow/sphinx_symbiflow_theme`

[mithro]

After fixing the issue around yosys no longer being found in the symbiflow conda channel the next error seems to be;

Ran pip subprocess with arguments:
[u'/home/docs/checkouts/readthedocs.org/user_builds/sphinxcontrib-hdl-diagrams/conda/84/bin/python', '-m', 'pip', 'install', '-U', '-r', '/home/docs/checkouts/readthedocs.org/user_builds/sphinxcontrib-hdl-diagrams/checkouts/84/tmpZZLQXZ.requirements.txt']
Pip subprocess output:
Collecting sphinx_symbiflow_theme
  Cloning http://github.com/SymbiFlow/sphinx_symbiflow_theme.git (to revision master) to /tmp/pip-install-m16aspc3/sphinx-symbiflow-theme_91a02a04c565470ca1d94cfad52b790e

Pip subprocess error:
  Running command git clone --filter=blob:none --quiet http://github.com/SymbiFlow/sphinx_symbiflow_theme.git /tmp/pip-install-m16aspc3/sphinx-symbiflow-theme_91a02a04c565470ca1d94cfad52b790e
  warning: redirecting to https://github.com/SymbiFlow/sphinx_symbiflow_theme.git/
  WARNING: Did not find branch or tag 'master', assuming revision or ref.
  Running command git checkout -q master
  error: pathspec 'master' did not match any file(s) known to git.
  error: subprocess-exited-with-error
  
  × git checkout -q master did not run successfully.
  │ exit code: 1
  ╰─> See above for output.
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× git checkout -q master did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

[mithro]

Testing two potential fixes;

• 013c5b3
• mithro@c8bf315