FEAT: share unsrt_et_al style as extension

[redeboer]

Imported this code and made it available as a Sphinx extension.

Usage

In your Sphinx configuration file (conf.py), add "sphinx_pybtex_etal_style" to your extensions:

extensions = [
    "sphinx_pybtex_etal_style",
]

and set your default bibliography style to "unsrt_et_al":

bibtex_default_style = "unsrt_et_al"

Alternatively, you can use the style for one bibliography only by specifying it in the .. bibliography:: directive:

.. bibliography:: /references.bib
  :style: unsrt_et_al