Make check-doc.py compliant to python3

Summary:
check_output returns bytes but the regex is using a string.
The decode() method will convert from bytes to string.

Depends on D2305

Test Plan:

python2 ./test/lint/check-doc.py
python3 ./test/lint/check-doc.py

Check the outputs are the same

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2306