chore(build): add MANIFEST.in to ignore unncessary directories

[brettlangdon]

We are including unnecessary directories in our source distributions. This PR adds a MANIFEST.in to help avoid adding these files.

I also considered adding prune tests/, since I doubt anyone would/could run our test suite from our source distribution alone anyways, and would mean pruning all the snapshot files. THoughts?

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

MANIFEST.in                                                             @DataDog/apm-core-python

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-12-20 19:42:00

Comparing candidate commit b4d1015 in PR branch brettlangdon/manifest with baseline commit 3d08709 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 394 metrics, 2 unstable metrics.