Only run dependabot on the "main" requirements.txt

[mdboom]

You can't exclude directories for dependabot, only exclude them, (thanks @hugovk for discovering that), so by moving the main requirements.txt to a directory that doesn't include all of the benchmarks' requirements.txt beneath it, we can limit dependabot to just that one file.

[hugovk]

Hmm, after merge, https://github.com/python/pyperformance/runs/13917712436 reports it's invalid:

Your .github/dependabot.yml contained invalid details Dependabot encountered the following error when parsing your .github/dependabot.yml:

The property '#/updates/0' did not contain a required property of 'schedule' Please update the config file to conform with Dependabot's specification.

Let's add a schedule value:

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#configuration-options-for-dependabotyml

[hugovk]

Please see https://github.com/python/pyperformance/pull/298 as a followup.