chore(ci): force setuptools_scm to use guess-next-dev version scheme in 1.19 branch (#6914)

To make system-tests pass in the 1.19 branch, this switches the
setuptools_scm version_scheme to guess-next-dev.

In the CI environment where the repo is in a detached HEAD state that
the branch's commit, guess-next-dev will properly guess the version as
1.19 instead of 1.20.

eg:
```
(setuptools-test) romain.komorn@COMP-CLXFH7L2J3 dd-trace-py % git checkout 5a1343e
Note: switching to '5a1343ec68fd8b8c38abfd4cfca891dc5dc6d19d'.

You are in 'detached HEAD' state. You can look around, make experimental
```
```
(setuptools-test) romain.komorn@COMP-CLXFH7L2J3 dd-trace-py % python -m setuptools_scm
...
1.19.0rc2.dev21+g5a1343ec6
```

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Author: romainkomorn-exdatadog

pyproject.toml

@@ -76,7 +76,7 @@ Homepage = "https://github.com/DataDog/dd-trace-py"
 "Source Code" = "https://github.com/DataDog/dd-trace-py/"
 
 [tool.setuptools_scm]
-version_scheme = "release-branch-semver"
+version_scheme = "guess-next-dev"
 write_to = "ddtrace/_version.py"
 
 [tool.isort]