fix: Remove ResourceWarning on module import

[connesy]

Checklist

• [X] I acknowledge that all my contributions will be made under the project's license
• [X] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [X] I have read the [Contribution Guide] and my PR follows them.
• [X] I updated my branch with the development branch.
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation about the functionality in the appropriate .md file
• [ ] I have added in line documentation to the code I modified

Short description of what this PR does:

Removes ResourceWarning on module import and from setup.py:

$ python -Wd
>>> import python_http_client
/home/<user>/miniconda3/envs/dev/lib/python3.7/site-packages/python_http_client/__init__.py:22: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/<user>/miniconda3/envs/dev/lib/python3.7/site-packages/python_http_client/VERSION.txt' mode='r' encoding='UTF-8'>
  __version__ = open(os.path.join(dir_path, 'VERSION.txt')).read().strip()
ResourceWarning: Enable tracemalloc to get the object allocation traceback

If you have questions, please send an email to the Twilio SendGrid Developer Experience Team, or file a GitHub Issue in this repository.

[SendGridDX]

All committers have signed the CLA.