Pin pylint to 2.4.* (#167)

Version 2.5.0 introduced a bug with the multiprocessing option. This
happens often enough that we should have pylint pinned and only upgrade
when we choose to do it. See pylint-dev/pylint#3524

Author: leouieda

.azure-pipelines.yml

@@ -27,7 +27,7 @@ jobs:
     vmImage: 'ubuntu-16.04'
 
   variables:
-    CONDA_INSTALL_EXTRA: "black flake8 pylint"
+    CONDA_INSTALL_EXTRA: "black flake8 pylint==2.4.*"
     PYTHON: '3.7'
 
   steps:

environment.yml

@@ -14,6 +14,6 @@ dependencies:
     - coverage
     - black
     - flake8
-    - pylint
+    - pylint=2.4.*
     - sphinx=2.2.1
     - sphinx_rtd_theme=0.4.3