auto-initialize to become opt-in on PyO3 0.14

[davidhewitt]

PR #1347 added the auto-initialize feature.

For compatibility in the 0.13 release series, this feature had to be enabled by default (where this behaviour already exists).

In addition, the compile_error error message where this feature is enabled for PyPy and statically-linked Python interpreters had to be supressed for feature = "extension-module" cases, to avoid breaking builds.

In PyO3 0.14 we should:

• remove the auto-initialize feature from default-features. (and update the guide)
• change the compile_error message in gil.rs to show always (not just for not(feature = "extension-module")).