MNT: Remove beautifulsoup4 from docs requirements

beautifulsoup4 was not used in our docs build. It is brought in
by pydata-sphinx-theme, but leave the dependency there. We do
use beautifulsoup4 in SRTM tiles, but that has nothing to do
with the docs.

docs/doc-requirements.txt

@@ -1,4 +1,3 @@
 sphinx
 sphinx-gallery
-beautifulsoup4
 pydata-sphinx-theme

environment.yml

@@ -27,7 +27,6 @@ dependencies:
   - pytest-mpl
   - pytest-xdist
   # Documentation
-  - beautifulsoup4
   - pydata-sphinx-theme
   - sphinx
   - sphinx-gallery

pyproject.toml

@@ -49,10 +49,11 @@ dependencies = [
 dynamic = ["version"]
 
 [project.optional-dependencies]
-doc = ["beautifulsoup4", "pydata-sphinx-theme", "sphinx", "sphinx-gallery"]
+doc = ["pydata-sphinx-theme", "sphinx", "sphinx-gallery"]
 speedups = ["pykdtree", "fiona"]
 ows = ["OWSLib>=0.20.0", "pillow>=6.1.0"]
 plotting = ["pillow>=6.1.0", "scipy>=1.3.1"]
+srtm = ["beautifulsoup4"]
 test = ["pytest>=5.1.2", "pytest-mpl>=0.11", "pytest-xdist", "pytest-cov", "coveralls"]
 
 [project.scripts]