MINOR: [C#] Bump pythonnet from 3.0.2 to 3.0.3 in /csharp

Bumps [pythonnet](https://github.com/pythonnet/pythonnet) from 3.0.2 to 3.0.3.
- [Release notes](https://github.com/pythonnet/pythonnet/releases)
- [Changelog](https://github.com/pythonnet/pythonnet/blob/master/CHANGELOG.md)
- [Commits](pythonnet/pythonnet@v3.0.2...v3.0.3)

---
updated-dependencies:
- dependency-name: pythonnet
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

csharp/test/Apache.Arrow.Tests/Apache.Arrow.Tests.csproj

@@ -17,7 +17,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
     <PackageReference Include="xunit.skippablefact" Version="1.4.13" />
-    <PackageReference Include="pythonnet" Version="3.0.2" />
+    <PackageReference Include="pythonnet" Version="3.0.3" />
   </ItemGroup>
 
   <ItemGroup>