Bugfix: Request text mode to Popen to avoid buffering warning (python 3.8)

[david-sinuela-pix4d]

Changelog: Fix: Avoid warning in "detect" process with Python 3.8, due to Popen with bufsize=1
Docs: omit

Closes #6332

This is a possible solution to the issue, the other one is to remove the bufsize argument but seeing that the code parses line by line this seems more appropriate.

From the commit message:

Python 3.8 started reporting warnings when binary files are opened with
bufsize > 0, see https://bugs.python.org/issue32236 for the change and
benoitc/gunicorn#2091 for a similar issue.

The error message was:

py.warnings: WARNING: /usr/lib/python3.8/subprocess.py:844: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used

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[memsharded]

I am testing this locally too on Windows. Why adding the universal_lines instead of dropping the bufsize=1? It also seems to work, and the offender is apparently the bufsize

[david-sinuela-pix4d]

Both would work I think, the lines below do line-wise processing that's why I preferred this approach but I can change it. Let me know.

[memsharded]

Sure, lets merge it, many thanks!