Move dev dependencies to requirements-dev.txt (#61)

[mehaase]

I used pip-tools to generate a requirements file. Also added a
Makefile to emulate how the requests package is laid out. Added
some documentation for using pip-tools. Finally, removed the
attrs dependency which wasn't really being used heavily enough
to justify its inclusion on this project.

[mehaase]

This isn't working as intended, yet. It is supposed to install trio-websocket in the development environment, so I put -e . in the dependencies file, but pip-compile expands this to a full path that only exists on my machine -e file:///Volumes/Case_Sensitive/trio-websocket. There's an issue filed on pip-tools but it looks like it's a complex interaction between pip and pip-tools that isn't likely to be solved anytime soon. I'll take another shot at this tomorrow.

[belm0]

It's certainly possible to get by without using "-e .". Just prefix commands with appropriate setting of PYTHONPATH.

test:
	PYTHONPATH=. pytest --cov=trio_websocket

[coveralls]

Pull Request Test Coverage Report for Build 81

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 91.644%

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Totals
Change from base Build 71:	0.5%
Covered Lines:	340
Relevant Lines:	371

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💛 - Coveralls

[mehaase]

This still doesn't work for RTD builds. RTD does not use the Makefile, so it does not know where trio-websocket is. RTD can be configured to install from setup.py, but then it installs packages that conflict with requirements-dev.txt. I've pushed a quick fix on master that abandons pip tools and uses a hand-written requirements-dev.txt instead.