Dont install pyface from source after 7.1.0 release

[rahulporuri]

Once pyface 7.1.0 is released (currently underway), it would be nice to stop installing pyface from source - and instead use the released version of pyface.

See https://github.com/enthought/traitsui/blob/master/ci-src-requirements.txt and

traitsui/etstool.py

Lines 112 to 116 in cfa7781

	# Required runtime dependencies. Should match install_requires in setup.py
	dependencies = {
	"traits" + TRAITS_VERSION_REQUIRES,
	# pyface is installed from source via ci-src-requirements.txt
	}