Bump nunit from 3.13.0 to 3.13.1 (#8)

Bumps [nunit](https://github.com/nunit/nunit) from 3.13.0 to 3.13.1.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/master/CHANGES.md)
- [Commits](nunit/nunit@v3.13...v3.13.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

MimeTypes.Tests/MimeTypes.Tests.csproj

@@ -11,7 +11,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="nunit" Version="3.13.0" />
+        <PackageReference Include="nunit" Version="3.13.1" />
         <PackageReference Include="NUnit3TestAdapter" Version="3.17.0" />
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />
     </ItemGroup>