Allow env=None to load secrets from system environment

[hartikainen]

Thanks for open-sourcing the package! I just gave it a spin and it looks promising. I found the use of dotenv a bit clumsy and implemented a workaround to use system environment variables. I thought I'd open a PR here in case someone else also finds it useful.

The new logic is as follows:

• if env is not None, then load_env_variables will load the variables from path env with dotenv.get_key,
• if env is None, then load_env_variables will load the variables from system environment.

Both cases will check that both environment variables exist.

Also, I was wondering: would it make sense to call these ONSHAPE_{ACCESS,SECRET}_KEY instead of just {ACCESS,SECRET}_KEY?