Add target netcoreapp3.1 to reduce dependencies in dotnetcore 3.1 projects

Author: Lukas Grützmacher

src/MessagePack/MessagePack.csproj

@@ -1,10 +1,10 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
     <NoWarn>$(NoWarn);CS0649</NoWarn>
     <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
-    <DefineConstants Condition=" '$(TargetFramework)' == 'netcoreapp2.1' ">$(DefineConstants);SPAN_BUILTIN</DefineConstants>
+    <DefineConstants Condition=" '$(TargetFramework)' != 'netstandard2.0' ">$(DefineConstants);SPAN_BUILTIN</DefineConstants>
     <CheckForOverflowUnderflow>True</CheckForOverflowUnderflow>
     <LangVersion>8.0</LangVersion>
 
@@ -34,6 +34,8 @@
 
   <ItemGroup>
     <PackageReference Include="Roslyn.Diagnostics.Analyzers" Version="2.9.7" PrivateAssets="all" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)'!='netcoreapp3.1'">
     <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="1.0.0" />
     <PackageReference Include="System.Collections.Immutable" Version="1.5.0" />
     <PackageReference Include="System.Reflection.Emit" Version="4.6.0" />

tests/MessagePack.Tests/MessagePack.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net472;netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>net472;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <LangVersion>8.0</LangVersion>
     <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>