Fix encoding errors in mikrotik device tracker

[bdurrer]

Description:

This PR fixes a bug where devices and/or networks with non-ascii characters crash the mikrotik device tracker.

Related issue (if applicable): fixes #

Pull request in home-assistant.github.io with documentation (if applicable): no influence on docs

Example entry for configuration.yaml (if applicable):

device_tracker:
  - platform: mikrotik
    host: 10.10.10.10
    username: homeassistant
    password: badpassword

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[balloob]

Thanks! 🐬