Refine SMTP HELO/EHLO handling; update readme

Don't override client secret when using PKCE

Add support for PKCE code challenges; update docs

Add auth-email.com sponsor link

Improve documentation and example configurations

Merge branch 'client_credentials_password'

Add support for certificate credential authentication using JWT asser…

…tions (#247)

Make sure token `expires_in` values are integers

Closes #237

Import `certifi` certificates when packaged using pyinstaller

Fix security issue with renewing expired tokens

See: GHSA-9wgg-m99q-hhfc