jupytext.read needs to check format is not none before updating fmt

[Wilsontomass]

As of now if you pass a file path in string form to jupytext.read() it does not convert the format with jupytext.formats.long_form_one_format() before attempting to call fmt.update(), which will be a string, and raise an AttributeError. Changing the order of if statements should solve this

[mwouts]

Hello @Wilsontomass , I won't be available to investigate this before early September. But this sounds both interesting and easy to fix.

Maybe in the mean time you would like to contribute the suggested fix, together with an additional test (maybe in a file named tests/test_jupytext_read.py) that would call the read function in the way you would expect it to work?