diff --git a/.github/workflows/phono3py-pytest-conda.yml b/.github/workflows/phono3py-pytest-conda.yml
index 979c8401..15b5b4df 100644
--- a/.github/workflows/phono3py-pytest-conda.yml
+++ b/.github/workflows/phono3py-pytest-conda.yml
@@ -12,7 +12,7 @@ jobs:
         shell: bash -l {0}
     strategy:
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
 
     steps:
     - uses: actions/checkout@v3
diff --git a/doc/changelog.md b/doc/changelog.md
index d7fc680f..009929cc 100644
--- a/doc/changelog.md
+++ b/doc/changelog.md
@@ -2,6 +2,10 @@
 
 # Change Log
 
+## Jun-7-2024: Version 3.1.1
+
+- Minor fix related to typehint for python-3.8.
+
 ## Jun-7-2024: Version 3.1.0
 
 - Change to write forces in phono3py-yaml as default.
diff --git a/doc/conf.py b/doc/conf.py
index 17a295e4..b9600efd 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -58,9 +58,9 @@
 # built documents.
 #
 # The short X.Y version.
-version = "3.0"
+version = "3.1"
 # The full version, including alpha/beta/rc tags.
-release = "3.1.0"
+release = "3.1.1"
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
diff --git a/phono3py/api_phono3py.py b/phono3py/api_phono3py.py
index c2197b0e..f720965f 100644
--- a/phono3py/api_phono3py.py
+++ b/phono3py/api_phono3py.py
@@ -33,6 +33,7 @@
 # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
+from __future__ import annotations
 
 import copy
 from collections.abc import Sequence
diff --git a/phono3py/version.py b/phono3py/version.py
index 663dd683..6a98d983 100644
--- a/phono3py/version.py
+++ b/phono3py/version.py
@@ -34,4 +34,4 @@
 # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
-__version__ = "3.1.0"
+__version__ = "3.1.1"