bpo-39040: added whitespaced to linesep_splitter in email.policy

[manfred-kaiser]

I'm working on a mailfilter in python and used the method "get_filename" of the "EmailMessage" class.

In some cases a wrong filename was returned. The reason was, that the Content-Disposition Header had a line break and the following intention was interpreted as part of the filename.
After fixing this bug, I was able to get the right filename.

https://bugs.python.org/issue39040

I had to change "linesep_splitter" in "email.policy" to match the intention.

Old Value:
linesep_splitter = re.compile(r'\n|\r')

New Value:
linesep_splitter = re.compile(r'\n\s+|\r\s+')

https://bugs.python.org/issue39040

[the-knights-who-say-ni]

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[bitdancer]

As noted in the issue this fix is incorrect.