📦 Fix setuptools-scm warnings on git archival (#1492)

Prior to this patch, `python -Im build` would emit warnings regarding
`.git_archival.txt` having not been templated that look as follows:

```console
$ python -Im build -vv 2>&1 | grep -i warni
/tmp/build-env-m3vufmd9/lib/python3.14/site-packages/setuptools_scm/git.py:427: UserWarning: git archive did not support describe output
  warnings.warn("git archive did not support describe output")
/tmp/build-env-m3vufmd9/lib/python3.14/site-packages/setuptools_scm/git.py:445: UserWarning: unprocessed git archival found (no export subst applied)
  warnings.warn("unprocessed git archival found (no export subst applied)")
/tmp/build-env-m3vufmd9/lib/python3.14/site-packages/setuptools_scm/git.py:427: UserWarning: git archive did not support describe output
  warnings.warn("git archive did not support describe output")
/tmp/build-env-m3vufmd9/lib/python3.14/site-packages/setuptools_scm/git.py:445: UserWarning: unprocessed git archival found (no export subst applied)
  warnings.warn("unprocessed git archival found (no export subst applied)")
```

This change addressed that by excluding said file from sdists produced
from within Git checkouts so the file only exists when exported into
an archive. `MANIFEST.in` itself is excluded due to a cascading
warning its presence introduces while fixing the original one.

ctx: jazzband/pip-tools#2261 (comment)
(cherry picked from commit 533c719)

Author: webknjaz

MANIFEST.in

@@ -1,3 +1,8 @@
+# do not include the `.git_archival.txt` file in sdist builds from repo source
+# this avoids setuptools-scm warnings on development builds
+exclude .git_archival.txt
 
-include README.md
-include LICENSE.md
+# furthermore do not include the `MANIFEST.in` file itself in sdist builds from
+# repo source to avoid the following setuptools warning on development builds:
+# warning: no previously-included files found matching '.git_archival.txt'
+exclude MANIFEST.in