net-im/telegram-desktop: fix build with GTK option

Telegram now includes a WebView feature when GTK is enabled, which
uses www/webkit2-gtk3.

While here, fix the following minor items:

• allow building with Python newer than 3.7
• devel/dee is not needed for build, remove
• clean up USES= syntax in the GTK option

Obtained from: https://github.com/desktop-app/lib_webview/commit/0b4100d7cecc4e748c51f3f51ebfd1392ec3978a

		https://github.com/desktop-app/lib_webview/commit/48d70e5258754b2415b07c88f0376212596e09a7

Approved by: Henry Hu <henry.hu.sh@gmail.com> (maintainer),

		arrowd (mentor), fluffy (mentor)

PR: 255719
Differential Revision: https://reviews.freebsd.org/D30224