fix: Strip trailing slash for repo url (#2495)

Strip potential trailing slash when building url on the case of relative
path without up-references. In particular, this fixes using
`experimental_index_url` with a AWS CodeArtifact python repository,
which currently fails package downloads due to an incorrect URL (with
double `//`) being produced by this code.

Co-authored-by: Richard Levasseur <[email protected]>

Author: amartani

python/private/pypi/parse_simpleapi_html.bzl

@@ -138,4 +138,4 @@ def _absolute_url(index_url, candidate):
         return "{}/{}".format(index_url, last.strip("/"))
 
     # relative path without up-references
-    return "{}/{}".format(index_url, candidate)
+    return "{}/{}".format(index_url.rstrip("/"), candidate)