graphics/impressive: update to 0.13.0-beta2 to build with python3

• the 0.13-beta versions introduce support for python 3

From upstream's news entries [1]:

2020-05-31Some maintenance in 0.13.0-beta2

The second beta version brings the expected bugfixes (mainly crashes on Python
3 due to oversights during porting), but also a few minor new features:
High-DPI scaling is now disabled on Windows, so that the display isn't as
blurry as it used to be on affected systems. New prev and next PageProps have
been introduced to change the playback order of pages. Finally, there's now a
»pseudo-binary« file called impressive in the .tar.gz release archive that's
more compact and with slightly quicker startup time than the standard source
code file.

[1] http://impressive.sourceforge.net/news.php

Approved by: kai (implicit)