python3Packages.requests: patch in CA bundles

The requests library defaults to using the certificates from the
certifi library when not otherwise specified.  If I understand the
discussion at #8247 correctly, we should instead patch it so that it
follows the following priority order:

1. the path pointed to by the environment variable $NIX_SSL_CERT_FILE

2. /etc/ssl/certs/ca-certificates.crt

3. whatever it was doing before (in this case, using certifi)

This commit implements that.

pkgs/development/python-modules/requests/0001-Prefer-NixOS-Nix-default-CA-bundles-over-certifi.patch

@@ -0,0 +1,60 @@
+From b36083efafec5a3c1c5864cd0b62367ddf3856ae Mon Sep 17 00:00:00 2001
+From: Keshav Kini <keshav.kini@gmail.com>
+Date: Sun, 16 May 2021 20:35:24 -0700
+Subject: [PATCH] Prefer NixOS/Nix default CA bundles over certifi
+
+Normally, requests gets its default CA bundle from the certifi
+package.  On NixOS and when using Nix on non-NixOS platforms, we would
+rather default to using our own certificate bundles controlled by the
+Nix/NixOS user.
+
+This commit overrides requests.certs.where(), which previously was
+just aliased to certifi.where(), so that now it does the following:
+
+- When run by Nix on non-NixOS, the environment variable
+  $NIX_SSL_CERT_FILE will point to the CA bundle we're using, so we
+  use that.
+
+- When running on NixOS, the CA bundle we're using has the static path
+  /etc/ssl/certs/ca-certificates.crt , so we use that.
+
+- Otherwise, we fall back to the original behavior of using certifi's
+  CA bundle.  Higher in the call stack, users of requests can also
+  explicitly specify a CA bundle to use, which overrides all this
+  logic.
+---
+ requests/certs.py | 18 +++++++++++++++++-
+ 1 file changed, 17 insertions(+), 1 deletion(-)
+
+diff --git a/requests/certs.py b/requests/certs.py
+index d1a378d7..faf462b7 100644
+--- a/requests/certs.py
++++ b/requests/certs.py
+@@ -12,7 +12,23 @@ If you are packaging Requests, e.g., for a Linux distribution or a managed
+ environment, you can change the definition of where() to return a separately
+ packaged CA bundle.
+ """
+-from certifi import where
++
++import os
++
++import certifi
++
++
++def where():
++    nix_ssl_cert_file = os.getenv("NIX_SSL_CERT_FILE")
++    if nix_ssl_cert_file and os.path.exists(nix_ssl_cert_file):
++        return nix_ssl_cert_file
++
++    nixos_ca_bundle = "/etc/ssl/certs/ca-certificates.crt"
++    if os.path.exists(nixos_ca_bundle):
++        return nixos_ca_bundle
++
++    return certifi.where()
++
+ 
+ if __name__ == '__main__':
+     print(where())
+-- 
+2.31.1
+

pkgs/development/python-modules/requests/default.nix

@@ -20,6 +20,8 @@ buildPythonPackage rec {
     sha256 = "sha256-J5c91KkEpPE7JjoZyGbBO5KjntHJZGVfAl8/jT11uAQ=";
   };
 
+  patches = [ ./0001-Prefer-NixOS-Nix-default-CA-bundles-over-certifi.patch ];
+
   postPatch = ''
     # Use latest idna
     substituteInPlace setup.py --replace ",<3" ""