[10070] Fix passing str command arguments to spawProcess with None encoding.

[adiroiban]

Scope and purpose

This tries to fix https://twistedmatrix.com/trac/ticket/10070

Default to utf-8 when encoding is None.

This is similar to twisted/towncrier#300

I am not a Py 3 expert, but my understand is that a standard stream has encoding None, when it is piped.

This is other info I found https://stackoverflow.com/questions/492483/setting-the-correct-encoding-when-piping-stdout-in-python

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10070
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• I have updated the automated tests.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.

[adiroiban]

Looks like there is another PR #1499 :)

[rodrigc]

@adiroiban Please stop working on this. I have closed Twisted bug 10070 and re-opened 10069. I mentioned here: https://twistedmatrix.com/pipermail/twisted-python/2020-December/065358.html that I am working on this problem, and collaborating with the Buildbot developers.

As I mentioned on the mailing list here: https://twistedmatrix.com/pipermail/twisted-python/2020-December/065365.html , I don't currently need your help on this, but will ask if I need any help from you.

Thanks.