Gensim optional and py 3.9 support

[avinashvarna]

• Fix #171 on py 3.9
• Make gensim installation optional. If gensim is not installed, a warning message is displayed and the lexical scoring will return a score proportional to the length of the split, so that shorter splits are weighted higher.

[avinashvarna]

CI passing on all three python versions 3.7/8/9 - https://travis-ci.com/github/avinashvarna/sanskrit_parser/builds/229445317

[kmadathil]

Looks good. Merging.