Add svg_stack package to Python virtual environment requirements, for…

… building SVG images. This is a small utility that can concatenate individual SVG files, replacing ImageMagick's montage (which does not work for SVG files).

requirements.txt

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+alabaster==0.7.12
+Babel==2.10.3
+certifi==2022.9.24
+charset-normalizer==2.1.1
+docutils==0.19
+idna==3.4
+imagesize==1.4.1
+importlib-metadata==5.0.0
+Jinja2==3.1.2
+lxml==4.9.3
+markdown-it-py==2.1.0
+MarkupSafe==2.1.1
+mdit-py-plugins==0.3.1
+mdurl==0.1.2
+myst-parser==0.18.1
+packaging==21.3
+Pygments==2.13.0
+pyparsing==3.0.9
+pytz==2022.4
+PyYAML==6.0
+requests==2.28.1
+six==1.16.0
+snowballstemmer==2.2.0
+Sphinx==5.3.0
+sphinxcontrib-applehelp==1.0.2
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==2.0.0
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.5
+svg-stack==0.1.0
+typing_extensions==4.4.0
+urllib3==1.26.12
+zipp==3.9.0