Pylint has been updated.

This commit fixes the following warnings:

```
rabbit/setup-guest-stage2:20:4: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
rabbit/setup-guest-stage2:22:4: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
rabbit/tests/load-balancing/configure-rabbit:7:0: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
setup.py:19:21: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
tests/integration/test_side_effects.py:31:15: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
tests/integration/test_side_effects.py:36:15: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
tests/integration/test_side_effects.py:41:15: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
tools/download-alpine-iso:33:13: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
```