Fix different behavior for jupyter vs. terminal

[qiangxinglin]

Submission Checklist

• [x] Run unit tests
• [x] Declare copyright holder and open-source license: see below

Summary

When run code in non-English system, jupyter would implicitly set encoding to 'utf-8', explicitly set the encoding here would make the behavior consistent.
Fix downstream bug https://github.com/facebook/prophet/issues/2462

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

myself

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)