Fix URL concatenation in anthropic/api.py

[nero19960329]

I noticed a small issue with the URL concatenation logic in

anthropic-sdk-python/anthropic/api.py

Line 71 in 4187c65

abs_url = urllib.parse.urljoin(self.api_url, path)

.
The current code uses urllib.parse.urljoin(self.api_url, path) to concatenate URLs, but this results in incorrect URLs when concatenating URLs like
http://localhost:5000/anthropic and /v1/complete, giving
http://localhost:5000/v1/complete instead of the expected
http://localhost:5000/anthropic/v1/complete.
A possible fix could be to directly concatenate the strings, like:
abs_url = "%s%s" % (self.api_base, url)
as implemented in the OpenAI API requestor:
https://github.com/openai/openai-python/blob/db3f3526f16ea518bcd16308508e035017f9259e/openai/api_requestor.py#L467

[nero19960329]

A possible fix PR: #27