[fix] #59 fix Python version in gitlab-ci

use Python 3.6.4

.gitlab-ci.yml

@@ -6,7 +6,7 @@ stages:
   - publish_doc
 
 variables:
-    PYENV_PYTHON_VERSION: 3.5.5
+    PYENV_PYTHON_VERSION: 3.6.4
 
 image: registry.duniter.org/docker/python3/duniterpy-builder:0.0.4
 
@@ -57,7 +57,6 @@ format:
     - .changes
   stage: format
   script:
-    - pyenv shell 3.6.4  # black install and run needs python 3.6.x minimum
     - pip install -r requirements_dev.txt
     - make check-format
 
@@ -76,7 +75,6 @@ check:
     - .changes
   stage: test
   script:
-    - pyenv shell 3.6.4  # black install needs python 3.6.x minimum
     - pip install -r requirements.txt
     - pip install -r requirements_dev.txt
     - make mypy
@@ -125,7 +123,6 @@ pages:
     - tags
     - master
   script:
-    - pyenv shell 3.6.4  # black install needs python 3.6.x minimum
     - pip install -r requirements.txt
     - pip install -r requirements_dev.txt
     - make docs