cargo: add `links = "python"`

PyOxidizer has crates depending on `pyo3` that would like to access
the `pyo3` crate configuration. (This use case isn't unique to
PyOxidizer.)

Cargo has a facility for enabling the build scripts of dependent
crates to access _exported_ variables via `DEP_` environment
variables. However, this only works if the exporting crate defines a
`links` key in its Cargo manifest. See
https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key.

While `pyo3`'s build script doesn't yet export the variables that
PyOxidizer will need, a prerequisite to making this work is adding
the `links` key. Since this change could introduce unintended
side-effects, it warrants being made in its own commit, which is
why we're making this change outside of #1793.

I _think_ this change should be mostly safe: the `links` key is
effectively metadata advertising that a crate links against a named
library. The only side-effects setting it has is to enable the
aforementioned `DEP_` environment variables in build scripts and
enforcing a limitation that only a single crate may link against the
same native library. I believe the only potential for this change
to cause problems is if there are multiple crates with `links =
"python"` entries. I'm not aware of any other crates that advertise
`links = "python"`: even `python3-sys` / `cpython` use `links =
"python3"` so this change should not prevent dual use of `pyo3` and
`cpython` in the same build.

CHANGELOG.md

@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Deprecate `PyTuple::slice`, new method `PyTuple::get_slice` added with `usize` indices. [#1828](https://github.com/PyO3/pyo3/pull/1828)
 - Deprecate FFI definitions `PyParser_SimpleParseStringFlags`, `PyParser_SimpleParseStringFlagsFilename`, `PyParser_SimpleParseFileFlags` when building for Python 3.9. [#1830](https://github.com/PyO3/pyo3/pull/1830)
 - Mark FFI definitions removed in Python 3.10 `PyParser_ASTFromString`, `PyParser_ASTFromStringObject`, `PyParser_ASTFromFile`, `PyParser_ASTFromFileObject`, `PyParser_SimpleParseStringFlags`, `PyParser_SimpleParseStringFlagsFilename`, `PyParser_SimpleParseFileFlags`, `PyParser_SimpleParseString`, `PyParser_SimpleParseFile`, `Py_SymtableString`, and `Py_SymtableStringObject`. [#1830](https://github.com/PyO3/pyo3/pull/1830)
+- `pyo3`'s `Cargo.toml` now advertises `links = "python"` to inform Cargo that it links against *libpython*. [#1819](https://github.com/PyO3/pyo3/pull/1819)
 
 ## [0.14.4] - 2021-08-29
 

Cargo.toml

@@ -12,6 +12,7 @@ categories = ["api-bindings", "development-tools::ffi"]
 license = "Apache-2.0"
 exclude = ["/.gitignore", ".cargo/config", "/codecov.yml", "/Makefile", "/pyproject.toml", "/tox.ini"]
 edition = "2018"
+links = "python"
 
 [dependencies]
 cfg-if = { version = "1.0" }