CI: exclude CI info from source tarballs

Debian-style packaging procedures generally want to add their own debian/ folder on top of orig.tar.gz

Note the overlap:

 * .gitattributes specifies what git-committed are to be ignored for git archive
 * MANIFEST.in specifies what python build places into sdist

One way to remove this overlap is deferring both decisions to the one git source of truth, e.g. by using setuptools-scm.

.gitattributes

@@ -0,0 +1,10 @@
+# note: edits here should cross-check with MANIFEST.in
+# note: setuptools-scm will prioritize MANIFEST.in over git attrs
+
+# someone wanting these would clone git instead of using source tarball
+.github export-ignore
+appveyor.yml export-ignore
+.readthedocs.yaml export-ignore
+
+# Debian prefers upstream source *without* packaging metadata
+.github/packaging/ export-ignore