Add automatic check for localization strings with multiple unnamed arguments

Improve localization support in etherlab (mostly)

cleanup etherlab: pylint, W0602 (global-variable-not-assigned) Using global for 'X' but no assignment is done

cleanup: pylint, W0109 # (duplicate-key) Duplicate key 'X' in dictionary

Add python3 syntax checking to CI

Fix old style exception raise and exec syntax

Use python 3 compatible exception syntax everywhere

cleanup: pylint, R0123 # (literal-comparison) Comparison to literal

Use predefined constants for PlcStatus instead of string literals

cleanup: pylint, R1704 # (redefined-argument-from-local) Redefining argument with the local name 'Y'

cleanup: pylint, R1701 # (consider-merging-isinstance) Consider merging these isinstance calls to isinstance(CTNLDFLAGS, (str, unicode))

cleanup: pylint, E1304 # (missing-format-string-key) Missing key 'X_name' in format string dictionary

cleanup: pylint, E1300 # (bad-format-character) Unsupported format character '"' (0x22) at index 17

cleanup: pylint, W0111 # (assign-to-new-keyword) Name async will become a keyword in Python 3.7

cleanup: pylint, E1310 (bad-str-strip-call) Suspicious argument in str.strip call

Fix segmentation fault when last lines in text POUs should be highlighted