port api/pushtx.py to Python 3

[HrachShah]

Summary

api/pushtx.py still uses five Python 2 idioms that prevent the file from being imported on Python 3 (and therefore prevent the entire WSGI app from starting, since msc_apps.py imports this module transitively):

• import urlparse — urlparse was renamed to urllib.parse in Python 3. The import is unused in this file (the only urlparse. call in the whole api/ package is in msc_apps.py, which still has its own copy of the same broken import) so it can simply be removed.
• import commands — commands was removed in Python 3 in favor of subprocess. The import is only there for the (commented-out) commands.getoutput(...) call, so it can also be removed.
• response_dict.has_key(field) — has_key was removed from dict in Python 2.3. Use field in response_dict.
• Three print statements without parentheses — print is a function in Python 3.
• except KeyError, e: — the comma form was removed in Python 3; the parser tries to evaluate e as an expression and raises SyntaxError.
• One bare except: on the error_codez lookup — narrows to except KeyError: so a non-dict return value or AttributeError surfaces instead of being silently masked.

This is the same Python 2 → 3 migration that earlier sessions applied to armory_service.py, pending.py, msc_apps.py, and rpcclient.py in the api package. msc_apps.py was fixed for the SyntaxError but only the offending lines were addressed; pushtx.py was not touched, and the WSGI process still refused to start because msc_apps.py imports the whole api/ package on startup.

Testing

python3 -c 'import ast; ast.parse(open("api/pushtx.py").read()) reports no SyntaxError. The file now parses under Python 3 and no longer references urlparse, commands, has_key, or the Python 2 except clause form.