Make pathos optional dependency

[mercuree]

Pathos is used for translate_html function only, but it requires bunch of own dependencies:

• pathos [required: >=0.2.9, installed: 0.2.9]
  • dill [required: >=0.3.5.1, installed: 0.3.5.1]
  • multiprocess [required: >=0.70.13, installed: 0.70.13]
    • dill [required: >=0.3.5.1, installed: 0.3.5.1]
  • pox [required: >=0.3.1, installed: 0.3.1]
  • ppft [required: >=1.7.6.5, installed: 1.7.6.5]
    • six [required: >=1.7.3, installed: 1.16.0]

Because pathos module is imported at the beginning of apis.py, it will break a whole code if I try to remove pathos and its deps.

[UlionTse]

@mercuree It Has little effect. If you think some modules have been updated, you can restore the updated modules after downloading.