Update cffi dep to 1.15.0

.github/workflows/tests.yml

@@ -12,6 +12,6 @@ jobs:
       with:
         python-version: '3.x'
     - name: Install dependencies
-      run: pip install -r requirements.txt
+      run: pip install --upgrade pip && pip install -r requirements.txt
     - name: Run tests
       run: ./run test

requirements.txt

@@ -2,7 +2,7 @@ attrs==19.3.0
 beautifulsoup4==4.10.0
 cachelib==0.4.1
 certifi==2020.4.5.1
-cffi==1.13.2
+cffi==1.15.0
 chardet==3.0.4
 click==8.0.3
 cryptography==3.3.2