chore(deps): make Qt backend optional (#350)

* chore(deps): make Qt backend optional

TODO:
- [ ] Add relevant entry in CHANGELOG
- [ ] Update install documentation
- [ ] Make sure `manim-slides convert` can run without any Qt backend
- [ ] Make sure test suite works (partially) without any Qt backend
- [ ] Make sure we can import `manim_slides` without any Qt backend

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* chore(deps): some fixes but wip

* chore(docs): update

* chore(deps): support PyQt6

* chore(deps): make Qt backend optional

TODO:
- [ ] Add relevant entry in CHANGELOG
- [ ] Update install documentation
- [ ] Make sure `manim-slides convert` can run without any Qt backend
- [ ] Make sure test suite works (partially) without any Qt backend
- [ ] Make sure we can import `manim_slides` without any Qt backend

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* chore(deps): some fixes but wip

* chore(docs): update

* chore(deps): support PyQt6

* fix(deps): ci and docs

* fix(lib): missing package

* chore(ci): does it work?

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* chore(test): skip failing

* chore(docs): update

* chore(docs): update

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix(docs): typo

* fix(test): quit instead of shutdown

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_present.py

@@ -3,7 +3,7 @@ from typing import Iterator, Tuple
 
 import pytest
 from click.testing import CliRunner
-from PySide6.QtWidgets import QApplication
+from qtpy.QtWidgets import QApplication
 
 from manim_slides.present import present
 
@@ -11,12 +11,12 @@ from manim_slides.present import present
 @pytest.fixture(autouse=True)
 def auto_shutdown_qapp() -> Iterator[None]:
     if app := QApplication.instance():
-        app.shutdown()
+        app.quit()
 
     yield
 
     if app := QApplication.instance():
-        app.shutdown()
+        app.quit()
 
 
 @pytest.fixture(scope="session")