Add Python version to rbt --version output.
sys.version has full python version value. The output is

Python 2.7.17 (default, Jan 10 2020, 00:33:00) 
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)]
and sys.version_info is a key=value pair tuple. 

sys.version_info(major=2, minor=7,
micro=17, releaselevel='final', serial=0)
and sys.version_info[:3] will get the first 3 keys' values into a tuple. 

The output is (2, 7, 17)
To format the version for python such as x.x.x, I get each digit in the tuple.

Tested in terminal in reviewboard-dev virtual env and reviewboard-dev-rbt for python 2 and 3.

and reviewboard-4.0 virtual envs and reviewboard-4.0-rbt for python 2 and 3.
Create TestCase and pass successfully using ./tests/runtests.py rbtools.commands.tests.test_main and  ./tests/runtests.py rbtools.commands.tests.test_main:VersionCommandTests. Test number is #246