Reinstalled Python (system-wide, w/ add to PATH) and now sphinx is working. But, I got some new errors:
# Erase previous directories to make sure we're clean.
rm -rf dist
rm -rf build
make build-all-deps
make[1]: Entering directory '/cygdrive/c/Users/wingi/mnemosyne'
# Also rebuilds the docs and the translations.
cd mnemosyne/libmnemosyne/docs && make SPHINXBUILD=sphinx-build html
make[2]: Entering directory '/cygdrive/c/Users/wingi/mnemosyne/mnemosyne/libmnemosyne/docs'
mkdir -p build/html build/doctrees
sphinx-build -b html -d build/doctrees source build/html
Running Sphinx v3.3.1
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 0 source files that are out of date
updating environment: 0 added, 1 changed, 0 removed
reading sources... [100%] index
C:\Users\wingi\mnemosyne\mnemosyne\libmnemosyne\docs\source\index.rst:62: WARNING: toctree contains reference to nonexisting document 'modules/language'
C:\Users\wingi\mnemosyne\mnemosyne\libmnemosyne\docs\source\index.rst:62: WARNING: toctree contains reference to nonexisting document 'modules/translator'
C:\Users\wingi\mnemosyne\mnemosyne\libmnemosyne\docs\source\index.rst:62: WARNING: toctree contains reference to nonexisting document 'modules/pronouncer'
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index
generating indices... genindex py-modindex done
writing additional pages... search done
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 3 warnings.
The HTML pages are in build\html.
Build finished. The HTML pages are in build/html.
cd mnemosyne/pyqt_ui && make clean
make[2]: Entering directory '/cygdrive/c/Users/wingi/mnemosyne/mnemosyne/pyqt_ui'
rm -f *~ *.pyc
rm -f ui_main_wdgt.py ui_add_cards_dlg.py ui_edit_card_dlg.py ui_review_wdgt.py ui_convert_card_type_keys_dlg.py ui_about_dlg.py ui_manage_plugins_dlg.py ui_manage_card_types_dlg.py ui_clone_card_type_dlg.py ui_configuration_wdgt_main.py ui_configuration_wdgt_card_appearance.py ui_activate_cards_dlg.py ui_card_set_name_dlg.py ui_configuration_dlg.py ui_criterion_wdgt_default.py ui_statistics_dlg.py ui_sync_dlg.py ui_configuration_wdgt_study.py ui_configuration_wdgt_servers.py ui_preview_cards_dlg.py ui_browse_cards_dlg.py ui_rename_tag_dlg.py ui_change_card_type_dlg.py ui_add_tags_dlg.py ui_remove_tags_dlg.py ui_getting_started_dlg.py ui_tip_dlg.py ui_import_dlg.py ui_compact_database_dlg.py ui_delete_unused_media_files_dlg.py ui_rename_card_type_dlg.py ui_export_dlg.py ui_export_metadata_dlg.py ui_edit_M_sided_card_type_dlg.py ui_edit_M_sided_card_template_wdgt.py ui_pronouncer_dlg.py ui_translator_dlg.py
rm -f mnemosyne_rc.py
make[2]: Leaving directory '/cygdrive/c/Users/wingi/mnemosyne/mnemosyne/pyqt_ui'
cd mnemosyne/pyqt_ui && make
make[2]: Entering directory '/cygdrive/c/Users/wingi/mnemosyne/mnemosyne/pyqt_ui'
python3 ./pyuic5 main_wdgt.ui > ui_main_wdgt.py
Traceback (most recent call last):
File "./pyuic5", line 15, in <module>
from PyQt5 import QtCore
ModuleNotFoundError: No module named 'PyQt5'
make[2]: *** [makefile:28: ui_main_wdgt.py] Error 1
make[2]: Leaving directory '/cygdrive/c/Users/wingi/mnemosyne/mnemosyne/pyqt_ui'
make[1]: *** [makefile:43: build-all-deps] Error 2
PyQt5 is most definitely installed.