Fix the Sphinx html_last_updated_fmt for Python3.
The html_last_updated_fmt option is interpeted as a
byte string in python3, causing Sphinx build to break.
This patch makes it utf-8 string.
Change-Id: I006b9cfef15a9e04c1e326d78dd30d2f5ad647c7
Closes-Bug:#1693670
os.popen() is deprecated since version 2.6. Resolved with use of
subprocess module.
Change-Id: I9ad7eb571d491c7620a05b4c73851e32981b2a4e
Closes-Bug: #1529836