older flassger required package incompatibility

Hi Team, thanks for the great package!
We came across an issue where flassger 0.9.5 imports a flask/jinja version that in turn imports a version of markupsafe that has a breaking change (soft_unicode was removed, soft_str replaced it), which causes a hard fail. A current workaround for this is manually importing an older version of markupsafe, but we wanted to suggest a specific version being specified in requirements for flask/jinja2, to avoid the package breaking for apps using older versions.

Thanks in advance for your help!

Start by reviewing the dependency requirements for flasgger 0.9.5 and reproduce the MarkupSafe import failure with the affected Flask/Jinja versions described in the issue. The work is done when the package requirements prevent the incompatible dependency combination and an installation or import check confirms the hard failure no longer occurs.

Written by the indexing model from the issue text.