HADOOP-15047. Python is required for -Preleasedoc but not documented in branch-2.8. Contributed by Bharat Viswanadham.

(cherry picked from commit a9f0536cedc1bd345a248c585e066df4218a0ac0)

BUILDING.txt

@@ -13,6 +13,7 @@ Requirements:
 * openssl devel (if compiling native hadoop-pipes and to get the best HDFS encryption performance)
 * Linux FUSE (Filesystem in Userspace) version 2.6 or above (if compiling fuse_dfs)
 * Internet connection for first build (to fetch all Maven and Hadoop dependencies)
+* python (for releasedocs)
 
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 The easiest way to get an environment with all the appropriate tools is by means