version: Fix setuptools_scm runtime error

`setuptools_scm` is used to retrieve version during runtime.
However, the folder where a package gets installed is not a git
repository which leads to `setuptools_scm` failure.

Use `get_distribution` instead of `setuptools_scm` to get version during
runtime.

Author: urutva

src/mbed_os_tools/test/host_tests_runner/__init__.py

@@ -20,6 +20,6 @@
 
 """
 
-from setuptools_scm import get_version
+from pkg_resources import get_distribution
 
-__version__ = get_version(root='../../../..', relative_to=__file__)
+__version__ = get_distribution("mbed-os-tools").version