Merge #11830: rpcuser.py: Use 'python' not 'python2'

6d2f277 rpcuser.py: Use 'python' not 'python2' (Henrik Jonsson)

Pull request description:

  Note that `rpcuser.py` seems to handle either version when called explicitly like `python2 rpcuser.py` / `python3 rpcuser.py`. This change allows the user's `python` to be used to interpret the script.

  There's not always a `python2` installed, e.g. if only Python 3.0+ is installed as `python` / `python3`, like on Arch Linux (https://github.com/hkjn/src/blob/master/bitcoin/Dockerfile#L14) but `python` is almost always present.

  # Tested

  Script is already compatible with both major versions:
  ```
  $ python2 share/rpcuser/rpcuser.py foobar
  String to be appended to bitcoin.conf:
  rpcauth=foobar:2951d04c215769c8887c1fa4a8f712c$63c9a08e3b69f811e3837c0d5b6a355b7f798afc7094d80008f5c56a056c387f
  Your password:
  TV4I54T6W38v1sj3iF4Xsw7A-wYav-Cn8uTr8qv4xZ8=

  $ python3 share/rpcuser/rpcuser.py foobar
  String to be appended to bitcoin.conf:
  rpcauth=foobar:129afbbd214c1f85fb6b9941402506f$4c5af73a2f3fd0a1d8232c28bc5c36f9b0cffd62b7b139beb328d089b16028dc
  Your password:
  qqbeWrGHqbYL1tUDh1wHKkejzxiGvOa3SPkDbbCwBfs=

  $ python2 --version
  Python 2.7.12

  $ python3 --version
  Python 3.5.2
  ```

Tree-SHA512: b5d6de15507cfd8dbb520325cf0b67fa471fa4fe3661de4ea6841fadb6ec9ba65d0c4f545f58578168c9ce9f6e483d613cce31ab3dd28117510bf9cada3b7b91

share/rpcuser/rpcuser.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2 
+#!/usr/bin/env python
 # Copyright (c) 2015-2016 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying 
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.