Implement Sections.get_or_create()

[nforro]

Fixes #404.

RELEASE NOTES BEGIN

There is a new convenience method Sections.get_or_create() that allows you to manipulate a section without checking if it exists first. If a section doesn't exist, it will be appended to the end. For example, this will work properly even on spec files without %changelog:

with spec.sections() as sections:
    changelog = sections.get_or_create("changelog")
    changelog[:] = ["%autochangelog"]

RELEASE NOTES END

[softwarefactory-project-zuul]

Build succeeded.
https://softwarefactory-project.io/zuul/t/packit-service/buildset/e6d4a773ab6844e399a4023c6e7ae82e

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[lachmanfrantisek]

Seems handy. Thanks for the example in the release notes.

[softwarefactory-project-zuul]

Build succeeded (gate pipeline).
https://softwarefactory-project.io/zuul/t/packit-service/buildset/fc79606bbb7d4d28b9614770736f4ae2

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