Avoid test failures with SSL proxies

Adds variables that do tell various libraries about custom CA used
by SSL proxies.

Avoids errors like:

   Could not fetch URL https://pypi.org/simple/mypy/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/mypy/ (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:

tox.ini

@@ -11,6 +11,10 @@ commands =
     coverage run -m pytest {posargs}
     coverage combine
     coverage report
+passenv =
+    CURL_CA_BUNDLE
+    REQUESTS_CA_BUNDLE
+    SSL_CERT_FILE
 
 [testenv:venv]
 deps =