GH-39942: [Python] Make capsule name check more lenient (#39977)

### Rationale for this change

While #39969 fixed the immediate issue caused by the update of the capsule name used by reticulate whilst converting an R "external pointer", it will still result in an error if somebody is using an older version of the Arrow R package.

### What changes are included in this PR?

The pyarrow Cython code was modified to accept capsules with the name NULL or "r_extptr".

### Are these changes tested?

Not sure where the best place for this is, but:

CRAN arrow + released pyarrow + new reticulate (errors):

``` r
library(arrow, warn.conflicts = FALSE)
reticulate::use_virtualenv("~/Desktop/rscratch/arrow/.venv")
packageVersion("arrow")
#> [1] '14.0.0.2'
packageVersion("reticulate")
#> [1] '1.35.0'
pa <- reticulate::import("pyarrow")
pa[["__version__"]]
#> [1] "15.0.0"

reticulate::r_to_py(arrow::int32())
#> PyCapsule_GetPointer called with incorrect name
```

CRAN arrow + pyarrow from this PR + old reticulate:

``` r
library(arrow, warn.conflicts = FALSE)
reticulate::use_virtualenv("~/Desktop/rscratch/arrow/.venv")
packageVersion("arrow")
#> [1] '14.0.0.2'
packageVersion("reticulate")
#> [1] '1.34.0'
pa <- reticulate::import("pyarrow")
pa[["__version__"]]
#> [1] "16.0.0.dev92+geafcff7a5"

reticulate::r_to_py(arrow::int32())
#> DataType(int32)
```

CRAN arrow + pyarrow from this PR + new reticulate:

``` r
library(arrow, warn.conflicts = FALSE)
reticulate::use_virtualenv("~/Desktop/rscratch/arrow/.venv")
packageVersion("arrow")
#> [1] '14.0.0.2'
packageVersion("reticulate")
#> [1] '1.35.0'
pa <- reticulate::import("pyarrow")
pa[["__version__"]]
#> [1] "16.0.0.dev92+geafcff7a5"

reticulate::r_to_py(arrow::int32())
#> DataType(int32)
```

### Are there any user-facing changes?

No
* Closes: #39942

Lead-authored-by: Dewey Dunnington <[email protected]>
Co-authored-by: Dewey Dunnington <[email protected]>
Co-authored-by: Antoine Pitrou <[email protected]>
Co-authored-by: Joris Van den Bossche <[email protected]>
Signed-off-by: Dewey Dunnington <[email protected]>

python/pyarrow/types.pxi

@@ -15,7 +15,13 @@
 # specific language governing permissions and limitations
 # under the License.
 
-from cpython.pycapsule cimport PyCapsule_CheckExact, PyCapsule_GetPointer, PyCapsule_New, PyCapsule_IsValid
+from cpython.pycapsule cimport (
+    PyCapsule_CheckExact,
+    PyCapsule_GetPointer,
+    PyCapsule_GetName,
+    PyCapsule_New,
+    PyCapsule_IsValid
+)
 
 import atexit
 from collections.abc import Mapping
@@ -105,6 +111,7 @@ cdef void* _as_c_pointer(v, allow_null=False) except *:
     (the latter for compatibility with raw pointers exported by reticulate)
     """
     cdef void* c_ptr
+    cdef const char* capsule_name
     if isinstance(v, int):
         c_ptr = <void*> <uintptr_t > v
     elif isinstance(v, float):
@@ -114,7 +121,20 @@ cdef void* _as_c_pointer(v, allow_null=False) except *:
             "Arrow library", UserWarning, stacklevel=2)
         c_ptr = <void*> <uintptr_t > v
     elif PyCapsule_CheckExact(v):
-        c_ptr = PyCapsule_GetPointer(v, NULL)
+        # An R external pointer was how the R bindings passed pointer values to
+        # Python from versions 7 to 15 (inclusive); however, the reticulate 1.35.0
+        # update changed the name of the capsule from NULL to "r_extptr".
+        # Newer versions of the R package pass a Python integer; however, this
+        # workaround ensures that old versions of the R package continue to work
+        # with newer versions of pyarrow.
+        capsule_name = PyCapsule_GetName(v)
+        if capsule_name == NULL or capsule_name == b"r_extptr":
+            c_ptr = PyCapsule_GetPointer(v, capsule_name)
+        else:
+            capsule_name_str = capsule_name.decode()
+            raise ValueError(
+                f"Can't convert PyCapsule with name '{capsule_name_str}' to pointer address"
+            )
     else:
         raise TypeError(f"Expected a pointer value, got {type(v)!r}")
     if not allow_null and c_ptr == NULL: