diff --git a/.github/workflows/continuous-integration-workflow.yml b/.github/workflows/continuous-integration-workflow.yml
index 08ad409a..d7892966 100644
--- a/.github/workflows/continuous-integration-workflow.yml
+++ b/.github/workflows/continuous-integration-workflow.yml
@@ -62,12 +62,12 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v1
         with:
-          python-version: '3.8'
+          python-version: '3.9'
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install mypy==0.800
-          pip install .
+          pip install -r requirements-dev.txt
+          pip install mypy==0.910 types-toml types-requests
       - name: Run mypy check
         run: mypy -p stellar_sdk
 
@@ -80,7 +80,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v1
         with:
-          python-version: '3.7'
+          python-version: '3.9'
 
       - name: Install dependencies
         run: |
diff --git a/stellar_sdk/sep/mnemonic.py b/stellar_sdk/sep/mnemonic.py
index 3acbaf02..0b4434f9 100644
--- a/stellar_sdk/sep/mnemonic.py
+++ b/stellar_sdk/sep/mnemonic.py
@@ -46,7 +46,7 @@ def __init__(self, language: Union[str, Language] = Language.ENGLISH) -> None:
 
         super().__init__(language)
 
-    def to_seed(self, mnemonic: str, passphrase: str = "", index: int = 0) -> bytes:
+    def to_seed(self, mnemonic: str, passphrase: str = "", index: int = 0) -> bytes:  # type: ignore[override]
         if not self.check(mnemonic):
             raise ValueError(
                 "Invalid mnemonic, please check if the mnemonic is correct, "