Rename `pep440-rs` to `Readme.md` (#1014)

This is due to a bug in Maturin
(https://github.com/PyO3/maturin/pull/1915), so I'll just fix our setup
to work with existing versions.

Closes https://github.com/astral-sh/puffin/issues/991.

.github/workflows/build-binaries.yml

@@ -34,8 +34,6 @@ env:
 
 jobs:
   sdist:
-    # TODO(charlie): Enable source distribution builds.
-    if: false
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4

crates/pep440-rs/Cargo.toml

@@ -3,7 +3,7 @@ name = "pep440_rs"
 version = "0.3.12"
 description = "A library for python version numbers and specifiers, implementing PEP 440"
 license = "Apache-2.0 OR BSD-2-Clause"
-include = ["/src", "Changelog.md", "License-Apache", "License-BSD", "README.md", "pyproject.toml"]
+include = ["/src", "Changelog.md", "License-Apache", "License-BSD", "Readme.md", "pyproject.toml"]
 
 edition = { workspace = true }
 rust-version = { workspace = true }