certifi missing from requirements.txt but imported in code

[Iamrodos]

I'm getting a ModuleNotFoundError: No module named 'certifi' in environments where system CA certificates aren't available.

if not https_ctx.get_ca_certs():
    import warnings
    warnings.warn(...)
    import certifi

This was added in PR #344 (commit 548a2ec) to fix issue #162, but certifi was never added to requirements.txt.

Two options 1) put certifi into requirements.txt, or 2) wrapped in a try/except with a more helpful error message directing users to install it.

I will create a PR for updating requirements.txt