MAINT: Set minimum yapf for checkspecs to 0.27

[effigies]

Summary

make specs currently flails a bit from user to user, apparently because yapf has changed its formatting rules.

Additionally, we've been ignoring errors reported by make specs, probably because they weren't really clearly errors.

List of changes proposed in this PR (pull-request)

• Set a new minimum version of yapf >=0.27. I've verified that 0.27 and 0.28 both give the same result, while 0.26 gives the old result.
• Add extensions metadata to list of accepted metadata to reduce noise
• Remove exists metadata from non-file traits as well as mandatory=False metadata

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[effigies]

Any objections?

[mgxd]

Looks fine to me