Typing: correct annotations for Canvas.content

[penguinolog]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 7739283377

• -2 of 5 (60.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.004%) to 70.964%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/display/_raw_display_base.py	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
urwid/display/_raw_display_base.py	1	22.76%

Totals
Change from base Build 7739072744:	-0.004%
Covered Lines:	8769
Relevant Lines:	12448

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💛 - Coveralls