documentation build issue with python 3.8

[sbrunato]

The Build the docs CI step (tox -e docs) fails in GH actions, due to an issue with rasterio==1.3.11.

Collecting rasterio (from eodag==3.0.0b4.dev12+gf373d7b0)
  Downloading rasterio-1.3.11.tar.gz (413 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 413.1/413.1 kB 67.3 MB/s eta 0:00:00
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'error'
  error: subprocess-exited-with-error

  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [3 lines of output]
      <string>:22: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
      WARNING:root:Failed to get options via gdal-config: [Errno 2] No such file or directory: 'gdal-config'
      ERROR: A GDAL API version must be specified. Provide a path to gdal-config using a GDAL_CONFIG environment variable or use a GDAL_VERSION environment variable.
      [end of output]

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

See https://github.com/rasterio/rasterio/issues/3168

[sbrunato]

rasterio==1.3.11 python3.8 wheels are on PyPI now.