diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0fd6f09..eb4fd52 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,11 +20,9 @@ jobs:
       fail-fast: false
       # https://blog.jaraco.com/efficient-use-of-ci-resources/
       matrix:
-        python: ["3.8", "3.12"]
+        python: ["3.9", "3.12"]
         platform: ["ubuntu", "macos", "windows"]
         include:
-        - python: "3.9"
-          platform: "ubuntu"
         - python: "3.10"
           platform: "ubuntu"
         - python: "3.11"
diff --git a/dahlia/constants.py b/dahlia/constants.py
index be2297c..aa4df74 100644
--- a/dahlia/constants.py
+++ b/dahlia/constants.py
@@ -46,8 +46,7 @@
     "f": "37",
 }
 
-COLORS_4BIT = {
-    **COLORS_3BIT,
+COLORS_4BIT = COLORS_3BIT | {
     "8": "90",
     "9": "94",
     "a": "92",
diff --git a/poetry.lock b/poetry.lock
index 9b6fce6..ba565ef 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry 1.8.2 and should not be changed by hand.
+# This file is automatically @generated by Poetry 1.8.3 and should not be changed by hand.
 
 [[package]]
 name = "attrs"
@@ -30,9 +30,6 @@ files = [
     {file = "babel-2.15.0.tar.gz", hash = "sha256:8daf0e265d05768bc6c7a314cf1321e9a123afc328cc635c18622a2f30a04413"},
 ]
 
-[package.dependencies]
-pytz = {version = ">=2015.7", markers = "python_version < \"3.9\""}
-
 [package.extras]
 dev = ["freezegun (>=1.0,<2.0)", "pytest (>=6.0)", "pytest-cov"]
 
@@ -812,17 +809,6 @@ files = [
 [package.dependencies]
 six = ">=1.5"
 
-[[package]]
-name = "pytz"
-version = "2024.1"
-description = "World timezone definitions, modern and historical"
-optional = false
-python-versions = "*"
-files = [
-    {file = "pytz-2024.1-py2.py3-none-any.whl", hash = "sha256:328171f4e3623139da4983451950b28e95ac706e13f3f2630a879749e7a8b319"},
-    {file = "pytz-2024.1.tar.gz", hash = "sha256:2a29735ea9c18baf14b448846bde5a48030ed267578472d8955cd0e7443a9812"},
-]
-
 [[package]]
 name = "pyyaml"
 version = "6.0.1"
@@ -1168,5 +1154,5 @@ testing = ["big-O", "jaraco.functools", "jaraco.itertools", "jaraco.test", "more
 
 [metadata]
 lock-version = "2.0"
-python-versions = "^3.8"
-content-hash = "007b6674422d4675a320d831afb6a8a2f855089decb66b9c913cdc45b68b23f4"
+python-versions = "^3.9"
+content-hash = "72070cf6e332c54d57a9ecb2f3f530429c7f753e97b8202ed929c8a7a24ed7f9"
diff --git a/pyproject.toml b/pyproject.toml
index f1cddf5..4c42b07 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -9,7 +9,7 @@ repository = "https://github.com/dahlia-lib/dahlia"
 readme = "README.md"
 
 [tool.poetry.dependencies]
-python = "^3.8"
+python = "^3.9"
 
 [tool.poetry.group.dev.dependencies]
 mkdocs = "^1.6.0"