Drop Python 3.4 compatibility, include 3.8

PyYAML started to require at least 3.5. There is possibly a way to pick
an older version of it to fulfill its needs, but it's also not worth to
invest effort in this old version anymore.

Use that chance to include 3.8 in the support scope.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.travis.yml

@@ -2,10 +2,10 @@ language: python
 dist: xenial
 
 python:
-  - "3.4"
   - "3.5"
   - "3.6"
   - "3.7"
+  - "3.8"
 
 services:
   - docker

setup.py

@@ -68,9 +68,10 @@ setup(
         'License :: OSI Approved :: MIT License',
 
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
     ],
     keywords='OpenEmbedded bitbake development',
 
@@ -88,6 +89,6 @@ setup(
         'jsonschema>=2.5.0',
     ],
 
-    # At least python 3.4 is needed for asyncio:
-    python_requires='>=3.4',
+    # At least python 3.5 is needed by now for PyYAML:
+    python_requires='>=3.5',
 )