plastex version 2.0 dependency

[royanin]

During the installation process, plastex 2.0 is installed automatically, which is dependent on Python3, whereas latex2edx depends on python 2.7. While the installation completes without an error (with ptyhon 2.7, plastex 2.0), there is a runtime error coming from the version mismatch.

By installing an earlier version of plastex (e.g., 1.0), the package runs as expected. However, we have not checked if there are bugs fixed in the plastex package between 1.0 and 2.0, and if running an earlier version of plastex could affect latex2edx performance.