Add svgconverter for badges in documentation LaTeX

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Adds a converter for the SVG items present in our README (needed for PDF generation)

Does this PR introduce a breaking change?

Some dependencies are added.

Other information:

This should fix the following warnings on RtD:

resolving references...
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://img.shields.io/pypi/v/xclim.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://img.shields.io/conda/vn/conda-forge/xclim.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://readthedocs.org/projects/xclim/badge)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://badges.gitter.im/Ouranosinc/xclim.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://img.shields.io/github/license/Ouranosinc/xclim.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://img.shields.io/badge/fair--software.eu-%E2%97%8F%20%20%E2%97%8F%20%20%E2%97%8F%20%20%E2%97%8F%20%20%E2%97%8B-yellow)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://zenodo.org/badge/142608764.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://github.com/Ouranosinc/xclim/workflows/xclim/badge.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://results.pre-commit.ci/badge/github/Ouranosinc/xclim/master.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://coveralls.io/repos/github/Ouranosinc/xclim/badge.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://img.shields.io/badge/code%20style-black-000000.svg)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://bestpractices.coreinfrastructure.org/projects/6041/badge)
/home/docs/checkouts/readthedocs.org/user_builds/xclim/checkouts/latest/README.rst:: WARNING: a suitable image for latex builder not found: ['image/svg+xml'] (https://badges.gitter.im/Ouranosinc/xclim.svg)

[aulemahal]

I don't know who reads the doc in LaTeX, but if this makes it kawaï, I'm in.