Added pytest-timeout to test dependencies

[radarhere]

In

Pillow/.appveyor.yml

Line 46 in 638ba16

- '%PYTHON%\%EXECUTABLE% -m pip install pytest pytest-cov pytest-timeout'

and

Pillow/.ci/install.sh

Lines 34 to 36 in 638ba16

	python3 -m pip install -U pytest
	python3 -m pip install -U pytest-cov
	python3 -m pip install -U pytest-timeout

pytest-timeout is installed as a test dependency. However, it is not documented in Tests/README.rst. This PR fixes that.