Add support for setting custom TLS CA cert file paths with env vars.

[hjoukl]

Summary of changes

Respect the SSL_CERT_FILE, REQUESTS_CA_BUNDLE and CURL_CA_BUNDLE env vars,
if set, to allow setting custom TLS CA cert file paths with setuptools'
urllib-based HTTPS implementation in a common fashion.

Closes #1630

(probably also addresses/enables a workaround for #1320)

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details
• Changes have documentation

[jaraco]

Thanks for proposing these changes. Your work here is excellent, with a clean test, clean implementation, and good changelog entry. Unfortunately, some time has passed and in the meantime, #1830 has landed, relying on pip for setup_requires and addressing the issue. If it turns out there are still outstanding cases revealed in #1630 not addressed by #1830, we may want to revisit this change.

[hjoukl]

Thanks for the kind words. Much appreciated 🙂!