Raise minimal Python version to 3.5

Newer PyYAML versions require Python 3.5.

Python 3.4 is EOL for some time and even 3.5 reached EOL recently. So
just switch to that as minimum version.

.github/workflows/python.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.4, 3.7, 3.8]
+        python-version: [3.5, 3.7, 3.8]
 
     steps:
     - uses: actions/checkout@v2

setup.py

@@ -58,7 +58,6 @@ setup(
         "Operating System :: Microsoft :: Windows",
         "Operating System :: MacOS",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.4",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Topic :: Software Development :: Embedded Systems",