Conditionally import Qubes or Containers deps.

Solves issues like these:

	ImportError while loading conftest '/home/user/dangerzone/tests/conftest.py'.
	tests/__init__.py:8: in <module>
	    from dangerzone.document import SAFE_EXTENSION
	dangerzone/__init__.py:16: in <module>
	    from .gui import gui_main as main
	dangerzone/gui/__init__.py:28: in <module>
	    from ..isolation_provider.qubes import Qubes, is_qubes_native_conversion
	dangerzone/isolation_provider/qubes.py:15: in <module>
	    from ..conversion.pixels_to_pdf import PixelsToPDF
	dangerzone/conversion/pixels_to_pdf.py:16: in <module>
	    import fitz
	E   ModuleNotFoundError: No module named 'fitz'

For context see discussion in [1].

[1]: #622 (comment)

dangerzone/gui/__init__.py

@@ -23,9 +23,8 @@
 
 from .. import args, errors
 from ..document import Document
-from ..isolation_provider.container import Container
 from ..isolation_provider.dummy import Dummy
-from ..isolation_provider.qubes import Qubes, is_qubes_native_conversion
+from ..isolation_provider.qubes import is_qubes_native_conversion
 from ..util import get_resource_path, get_version
 from .logic import DangerzoneGui
 from .main_window import MainWindow
@@ -135,9 +134,13 @@ def gui_main(
         dummy = Dummy()
         dangerzone = DangerzoneGui(app, isolation_provider=dummy)
     elif is_qubes_native_conversion():
+        from ..isolation_provider.qubes import Qubes
+
         qubes = Qubes()
         dangerzone = DangerzoneGui(app, isolation_provider=qubes)
     else:
+        from ..isolation_provider.container import Container
+
         container = Container(enable_timeouts=enable_timeouts)
         dangerzone = DangerzoneGui(app, isolation_provider=container)