Respect `REQUESTS_CA_BUNDLE` environment variable

[thwllms]

Sitting behind a corporate VPN/firewall which performs HTTPS inspection / SSL certificate replacement, I have to configure my environment to deal with otherwise fishy-looking certificates issued by the system. By default the basic requests.get, requests.post, etc., functions look for an environment variable REQUESTS_CA_BUNDLE (see docs), which points to a custom certificate bundle file that includes the Root CA for my company's VPN/firewall system. That one setting solves a lot of issues in Python land.

pystac-client, however, doesn't seem to respect that environment variable. It uses raw Request and Session objects to set up requests, resulting in this error:

SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:997)

I think the send method call on this line should probably account for REQUESTS_CA_BUNDLE via the verify argument, something like this:

from requests import Request, Session
import os

session = Session()
request = Request(method="GET", url="https://example.com")
prepped = session.prepare_request(request)
response = session.send(prepped, verify=os.environ.get("REQUESTS_CA_BUNDLE"))