The `-zapwallettxes` functionality has been removed in v0.21.0
(see commit 3340dbadd3 / PR #19671),
with the parameter being kept as hidden option, to inform users via
an exit error that `abandontransaction` should be used instead.
As any guides that still suggest to use `-zapwallettxes` would refer to
a Bitcoin Core version that is EOL since many years (i.e. <= v0.20.x),
it is highly unlikely that the error caused by the option is still
relevant for any user, hence it seems fine to remove it now.
-zapwallettxes is made a hidden option to inform users that it is
removed and they should be using abandontransaction to do the stuck
transaction thing.
Flagged by flake8 v3.6.0, as W605, plus a few others identified
incidentally, e.g. 59ffecf66cf4d08c4b431e457b083878d66a3fd6.
Note that r"\n" matches to "\n" under re.match/search.
Removing the 'universal_newlines' and 'encoding' args from the subprocess.check_outputs fuction. 'universal_newlines' is supported in 3.4, but 'encoding' is not. Without specifying 'encoding' it will make a guess at encoding, which can break things on BSD systems. We must handle encoding/decoding ourselves until we can use Python 3.6
Make lint/check-doc.py Python 3.4 compatible.
Also add .python-version for pyenv which will cause tests with too
modern syntax to fail on developer machine rather than on Travis.
