updated dev-version to 3.5.2
translate extension group name in upload form
updated filter dropdown HTML
Use placeholder with version_generate, fixed #130
Merge pull request #129 from honi/stable/3.5.x

Add missing translation in upload form and update filter markup
minor docs update, fixed #139
templatetag exception, fixed #138
setup and python3, see #137
Merge remote-tracking branch 'remotes/sehmaschine/stable/3.5.x'