fetchurl: add testpypi mirror

[Ekleog]

This should make it easier to run nixpkgs-based tests from versions that
are still only on test-pypi, and should not cost anything significant.

nixpkgs-check report

version: nixpkgs-check v0.1.0 on NixOS 21.05, sandbox = "true"
packages declared changed: {"fetchurl"}
manual tests declared performed:

• ✔ built on NixOS
• 😢 not built on MacOS
• 😢 not built on Other Linux distributions
• building rss2email v3.13.1 from test-pypi
• running nixosTests.rss2email from that version

complies with contributing.md: ✔ yes
package fetchurl: 😢 still does not build

[r-rmcgibbo]

Result of nixpkgs-review pr 123146 at c02f66f run on aarch64-linux 1

1 package failed to build:

• dotnet-aspnetcore (dotnetCorePackages.aspnetcore_2_1)

Note that build failures may predate this PR, and could be nondeterministic or hardware dependent.
Please exercise your independent judgement. Does something look off? Please file an issue or reach out on IRC.