[test] Travis: enforce Python 3.4 support in functional tests

Make lint/check-doc.py Python 3.4 compatible.

Also add .python-version for pyenv which will cause tests with too
modern syntax to fail on developer machine rather than on Travis.

.python-version

@@ -0,0 +1 @@
+3.4.9

.travis.yml

@@ -43,7 +43,7 @@ jobs:
       env:
       cache: false
       language: python
-      python: '3.6'
+      python: '3.4' # Oldest supported version according to doc/dependencies.md
       install:
         - set -o errexit; source .travis/lint_04_install.sh
       before_script:

test/lint/check-doc.py

@@ -26,8 +26,12 @@
 
 
 def main():
-    used = check_output(CMD_GREP_ARGS, shell=True, universal_newlines=True, encoding='utf8')
-    docd = check_output(CMD_GREP_DOCS, shell=True, universal_newlines=True, encoding='utf8')
+    if sys.version_info >= (3, 6):
+        used = check_output(CMD_GREP_ARGS, shell=True, universal_newlines=True, encoding='utf8')
+        docd = check_output(CMD_GREP_DOCS, shell=True, universal_newlines=True, encoding='utf8')
+    else:
+        used = check_output(CMD_GREP_ARGS, shell=True, universal_newlines=True) # encoding='utf8'
+        docd = check_output(CMD_GREP_DOCS, shell=True, universal_newlines=True) # encoding='utf8'
 
     args_used = set(re.findall(re.compile(REGEX_ARG), used))
     args_docd = set(re.findall(re.compile(REGEX_DOC), docd)).union(SET_DOC_OPTIONAL)