Removed deprecated install-venv

[radarhere]

Helps python-pillow#6072

[hugovk]

Thanks, some suggestions!

[hugovk]

Let's redirect all these to /dev/null to avoid the error message:

python3 -c "import pytest" || python3 -m pip install pytest
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'pytest'
Collecting pytest
...

Suggested change

	python3 -c "import pytest" || python3 -m pip install pytest
	python3 -c "import pytest" > /dev/null 2>&1 || python3 -m pip install pytest

[radarhere]

Sure, done

[hugovk]

And when we have multiple we can remove duplication something like:

Suggested change

	html:
	python3 -c "import sphinx" || python3 -m pip install sphinx
	install-sphinx:
	python3 -c "import sphinx" > /dev/null 2>&1 || python3 -m pip install sphinx
	html: install-sphinx

[radarhere]

Ok, done

[hugovk]

Thanks!