chore: support build on git export

- Add .git_archival.txt template that substitute on export

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

.git_archival.txt

@@ -0,0 +1,4 @@
+node: $Format:%H$
+node-date: $Format:%cI$
+describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
+ref-names: $Format:%D$

.gitattributes

@@ -0,0 +1 @@
+.git_archival.txt  export-subst

MANIFEST.in

@@ -1,4 +1,5 @@
 include *.rst
+include *.txt
 include LICENSE
 include pyproject.toml
 include .flake8