_text_formats should include txt but not text

[dachengx]

What does the code in this PR do / what does it improve?

When judging whether the file is a text file in resource_from_url, the text file should end with txt but not text.

straxen/straxen/common.py

Line 275 in 476bf1a

is_binary = fmt not in _text_formats

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

[coveralls]

coverage: 91.337% (+0.02%) from 91.316%
when pulling 64ead4e on update_text_formats
into b18e8f3 on master.

[yuema137]

It's a straightforward fix. Thanks