Replace requirements.txt with extras

[hugovk]

Using extras_require means we can install just those requirements needed for a given task at the same time as installing Pillow, rather using a separate command that installs the whole bunch.

For example, to install things needed for testing:

pip install .[tests]

# Or my Mac terminal needs quotes
pip install ".[tests]"

Or for building docs:

pip install .[docs]

# Or my Mac terminal needs quotes
pip install ".[docs]"

---

This also removes two non-existent commands from the make help, and adds some capitalisation.

---

Whilst editing Makefile I noticed install-venv is deprecated but not in the deprecations docs:

	@echo "  install-venv       (deprecated) install in virtualenv"

...

.PHONY: install-venv
install-venv:
	echo "'install-venv' is deprecated and will be removed in a future Pillow release"
	virtualenv .

Deprecated in #5171 / #5159 / Pillow 8.1.0 / 2 Jan 2021, so strictly speaking would have been eligible for removal in 9.0.0 / Jan 2022.

But this is arguably a dev helper rather than a formal API. Shall we wait for Pillow 10 (and add it to the deprecations docs?), or is it safe to remove it now?

[radarhere]

Whilst editing Makefile I noticed install-venv is deprecated but not in the deprecations docs:

	@echo "  install-venv       (deprecated) install in virtualenv"

...

.PHONY: install-venv
install-venv:
	echo "'install-venv' is deprecated and will be removed in a future Pillow release"
	virtualenv .

Deprecated in #5171 / #5159 / Pillow 8.1.0 / 2 Jan 2021, so strictly speaking would have been eligible for removal in 9.0.0 / Jan 2022.

But this is arguably a dev helper rather than a formal API. Shall we wait for Pillow 10 (and add it to the deprecations docs?), or is it safe to remove it now?

You've suggested before that the Makefile is a lower priority, so I don't mind. See hugovk#81

I think the bigger change is removing install-req without deprecation - but I can imagine an argument that changes to how you compile Pillow are easier to adapt to than changes in the core API, and that if you're compiling Pillow yourself, there is a measure of acceptance that this might not be a completely stable experience.

Shall we make a note of these changes in the release notes at least?

[hugovk]

Yep, added to release notes in the last commit.