yum install fails with HTTP 403 trying to access r

I'm trying to install a package on a old Fedora 20 virtual machine.

yum install<the_package_name> results in a failure with an HTTP 403 error:

http://download.fedoraproject.org/<...(truncated)...>/repomd.xml:
[Errno 14] HTTP Error 403 - Forbidden

My web-browser can't see anything at http://download.fedoraproject.org/pub/fedora/linux/updates/20 so I realize FC20 is no more supported (EOL) and its repository URL has changed. So I fix the baseurl in /etc/yum.repos.d/fedora.repo to look like this:

baseurl=http://archives.fedoraproject.org/<...(truncated)...>

I'm sure the URL is now correct, because I can download repomd.xml using curl or wget, and access it in my web browser...

But yum install <the_package_name> continues to fail with an HTTP 403 error! It can't access repomd.xml at the correct URL:

http://archives.fedoraproject.org/<...(truncated)...>/repomd.xml:
[Errno 14] HTTP Error 403 - Forbidden

Can you help me overcome this issue and install packages on this old Fedora (FC 20)?

Note 1: I'm working from behind a proxy (not my choice).

Note 2: Upgrading my Fedora 20 to Fedora 21 or 22 is not an option either.

SOLVED

/etc/yum.conf: remove spurious line proxy=http://<wrong proxy IP address>
/etc/yum.repos.d/fedora.repo: update the Fedora repository URL
/etc/yum.repos.d/fedora-updates.repo: update the Fedora repository URL
yum clean metadata

yum install <the_package> works fine now, thanks!

Here are the suggestions (from Etan Reisner) that helped me solve the issue:

Check the proxy configuration in /etc/yum.conf
Check all YUM .repo files are using the up-to-date Fedora repo URL
Launch yum clean metadata to ensure YUM uses the updated .repo files contents
Try again yum install <the_package>