Converted most assert statements to pytest

[radarhere]

Helps #4193 with a different approach - rather than converting individual files completely, this PR changes single assert lines.

[hugovk]

And do the rest of the conversion in follow-up PRs? It's a bit more churny, but sure.

Perhaps omit test_file_bmp.py and test_file_im.py from here, as they have full conversion in #4424?

[radarhere]

I just thought it was an easier way to approach the repetitive task of conversion.

Okay, omitted those files.

[hugovk]

I restarted the failed AppVeyor builds after a network error:

Build started
ECHO %PYTHON%
C:/Python38-x64
git clone -q https://github.com/python-pillow/Pillow.git c:\pillow
git fetch -q origin +refs/pull/4443/merge:
git checkout -qf FETCH_HEAD
Restoring build cache
Cache 'C:\Users\appveyor\AppData\Local\pip\Cache' - Restored
Running Install scripts
curl -fsSL -o pillow-depends.zip https://github.com/python-pillow/pillow-depends/archive/master.zip
curl: (18) transfer closed with outstanding read data remaining
Command exited with code 18

If it happens again, we can look to hardening the curl.