Refactor is_valid to be an instance attribute

[ffe4]

Description

Refactoring of SpanContext.is_valid() to be a variable, rather than a method. This makes the API consistent with is_remote, which is defined similarly in the specification.

Since I am new to this project, I am not sure whether the removal of the method might break something downstream, but adding a deprecation warning would require also choosing a different name for the attribute.

Fixes #996

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated (where?)
• Unit tests have been updated
• Documentation has been removed (by removing the method)

[ffe4]

The build passed on my end, so it probably just needs a rerun.

[lzchen]

Changelog

[codeboten]

LGTM