Update web_base.py _fetch() method For SiteMapLoader (#6256)

A must-include for SiteMap Loader to avoid the SSL verification error.
Setting the 'verify' to False by ``` sitemap_loader.requests_kwargs =
{"verify": False}``` does not bypass the SSL verification in some
websites.

There are websites (https:// researchadmin.asu.edu/ sitemap.xml) where
setting "verify" to False as shown below would not work:
sitemap_loader.requests_kwargs = {"verify": False} 

We need this merge to tell the Session to use a connector with a
specific argument about SSL:
 \# For SiteMap SSL verification
if not self.request_kwargs['verify']:
    connector = aiohttp.TCPConnector(ssl=False)
else:
    connector = None
 
<!--
Thank you for contributing to LangChain! Your PR will appear in our
release under the title you set. Please make sure it highlights your
valuable contribution.

Replace this with a description of the change, the issue it fixes (if
applicable), and relevant context. List any dependencies required for
this change.

After you're done, someone will review your PR. They may suggest
improvements. If no one reviews your PR within a few days, feel free to
@-mention the same people again, as notifications can get lost.

Finally, we'd love to show appreciation for your contribution - if you'd
like us to shout you out on Twitter, please also include your handle!
-->

Fixes #5483 

#### Before submitting

<!-- If you're adding a new integration, please include:

1. a test for the integration - favor unit tests that does not rely on
network access.
2. an example notebook showing its use


See contribution guidelines for more information on how to write tests,
lint
etc:


https://github.com/hwchase17/langchain/blob/master/.github/CONTRIBUTING.md
-->

#### Who can review?

Tag maintainers/contributors who might be interested:

@hwchase17 
@eyurtsev

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

langchain/document_loaders/web_base.py

@@ -100,7 +100,13 @@ def web_path(self) -> str:
     async def _fetch(
         self, url: str, retries: int = 3, cooldown: int = 2, backoff: float = 1.5
     ) -> str:
-        async with aiohttp.ClientSession() as session:
+        # For SiteMap SSL verification
+        if not self.requests_kwargs.get("verify", True):
+            connector = aiohttp.TCPConnector(ssl=False)
+        else:
+            connector = None
+
+        async with aiohttp.ClientSession(connector=connector) as session:
             for i in range(retries):
                 try:
                     async with session.get(