Consistency: install_command configured in tox.ini should more closely correspond to the default tox install command

[jayaddison]

In particular, these lines don't include the -I (as in the first letter of Indigo) isolated-mode flag for Python, that tox includes by default.

recipe-scrapers/tox.ini

Lines 13 to 14 in baf3403

	install_command =
	py-darwin: python -m pip install --only-binary=lxml {opts} {packages}

(these lines introduced by https://github.com/hhursev/recipe-scrapers/pull/650/files#diff-ef2cef9f88b4fe09ca3082140e67f5ad34fb65fb6e228f119d3812261ae51449R14-R15)

To fix #698, one possible short-term solution could be to provide a install_command for the environment name py. That's not ideal, because it should be kept up-to-date with any changes to the tox install default, and could diverge if it doesn't (extra maintenance burden / tech debt). If we're doing that, we should match the current defaults.. and when inspecting those, I noticed that the py-darwin environment isn't consistent.

[jayaddison]

Resolved by #702.