build: Simplify dependency management

Make some previously optional dependencies required:

- pyyaml was always pulled in as transient dependency anyways.
- requests is probably always installed anyways, because pip needs it
- diskcache is relatively small and pure-Python, no need to guard it

This set of dependencies enables all currently implemented FileHandlers.
The only notable runtime-relevant extra is now "png" (and its alias
"termgraphics"), which uses cairosvg, which in turn depends on native
libraries that we can't expect to be installed (especially on Windows).

pyproject.toml

@@ -33,10 +33,13 @@ classifiers = [
   "Typing :: Typed",
 ]
 dependencies = [
+  "diskcache>=5.0",
   "lxml>=4.5.0",
   "markupsafe>=1.1",
   "Pillow>=7.0.0",
   "platformdirs>=1.4.1",
+  "pyyaml>=6.0",
+  "requests>=2.25.0",
   "svgwrite>=1.3.1",
   "typing_extensions >=4.0.0, <5",
 ]
@@ -51,34 +54,24 @@ docs = [
   "ipython",
   "jinja2",
   "nbsphinx",
-  "pyyaml>=6.0",
   "sphinx!=7.2.0,!=7.2.1,!=7.2.2",
   "sphinx-argparse-cli",
   "tomli; python_version<'3.11'",
 ]
 
 test = [
   "click",
-  "cssutils",
-  "diskcache>=5.0",
   "pytest",
   "pytest-cov",
-  "pyyaml>=6.0",
   "requests-mock",
 ]
 
 cli = [
   "click",
 ]
 
-decl = [
-  "pyyaml>=6.0",
-]
-
-httpfiles = [
-  "diskcache>=5.0",
-  "requests>=2.25.0",
-]
+decl = []
+httpfiles = []
 
 png = [
   "cairosvg>=2.5.2",