Recommend that packages pin extension-helpers in pyproject.toml?

[astrofrog]

Issues such as #70 mean that it's difficult for us to make any breaking changes in extension-helpers because e.g. astropy specifies extension-helpers with no pinning in pyproject.toml under the build dependencies. This is an issue because then if anyone tries to compile an old source release of astropy we will always need extension-helpers to be backward-compatible. We might want to start recommending some form of pin, even it is is just extension-helpers>1,<2, so that some day we can choose to pull the plug and make breaking changes.

Note that I know in many cases upper version pins are undesirable, but this is mostly true for run-time dependencies. For build-time dependencies I think there are a lot of benefits from doing so.