[AIRFLOW-2857] Fix Read the Docs env (#3703)

The Read the Docs build process was broken due to #3660. This PR fixes this.

.readthedocs.yml

@@ -6,9 +6,9 @@
 # to you under the Apache License, Version 2.0 (the
 # "License"); you may not use this file except in compliance
 # with the License.  You may obtain a copy of the License at
-# 
+#
 #   http://www.apache.org/licenses/LICENSE-2.0
-# 
+#
 # Unless required by applicable law or agreed to in writing,
 # software distributed under the License is distributed on an
 # "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
@@ -22,4 +22,4 @@ python:
         - doc
         - docker
         - gcp_api
-        - emr 
+        - emr

setup.py

@@ -37,6 +37,14 @@
 
 # See LEGAL-362
 def verify_gpl_dependency():
+    # The Read the Docs build environment [1] does a pip install of Airflow which cannot
+    # be overridden with custom environment variables, so we detect the READTHEDOCS env
+    # var they provide to set the env var that avoids the GPL dependency on install when
+    # building the docs site.
+    # [1]: http://docs.readthedocs.io/en/latest/builds.html#build-environment
+    if os.getenv("READTHEDOCS") == "True":
+        os.environ["SLUGIFY_USES_TEXT_UNIDECODE"] = "yes"
+
     if (not os.getenv("AIRFLOW_GPL_UNIDECODE")
             and not os.getenv("SLUGIFY_USES_TEXT_UNIDECODE") == "yes"):
         raise RuntimeError("By default one of Airflow's dependencies installs a GPL "
@@ -161,6 +169,7 @@ def write_version(filename=os.path.join(*['airflow',
 databricks = ['requests>=2.5.1, <3']
 datadog = ['datadog>=0.14.0']
 doc = [
+    'mock',
     'sphinx>=1.2.3',
     'sphinx-argparse>=0.1.13',
     'sphinx-rtd-theme>=0.1.6',