Zanshincli

What's changed
- Change how CLI get SDK version in order to be compliant with PEP standards
- Update httpx library to fix vulnerability

Commits
* getting sdk version from `zanshinsdk.__version__` by leonardoviveiros in https://github.com/tenchi-security/zanshin-cli/pull/22
* Bump ZanshinSDK version to 1.25.0 by leonardoviveiros in https://github.com/tenchi-security/zanshin-cli/pull/24


**Full Changelog**: https://github.com/tenchi-security/zanshin-cli/compare/1.2.6...1.2.7

Now you can use the following [Environment Variables](https://www.python-httpx.org/environment_variables/#proxies) to configure Zanshin CLI:

`ZANSHIN_API_KEY`: Will setup your Zanshin credentials
`ZANSHIN_API_URL`: Will define the API URL. Default is https://api.zanshin.tenchisecurity.com
`ZANSHIN_USER_AGENT`: If you want to overwrite the User Agent when calling Zanshin API
`HTTP_PROXY` | `HTTPS_PROXY`: Zanshin CLI uses HTTPX under the hood, checkout the Environment Variables section of their documentation for more use cases

- Adding the method `onboard_organization_aws_organization_scan_target` that supports onboard of AWS Organizations, that are multiple AWS Accounts.
- Drop support for python 3.6 and 3.7.

- Adding the method `onboard_organization_aws_scan_target` that supports onboard of AWS Scan Targets. For this to work, `boto3` is required.
- Boto3 is a new dependency of Zanshin CLI.