Add the netcoreapp3.1 TFM and bump Mono.Unix version (#93)

Author: grendello

LibZipSharp.props

@@ -1,9 +1,9 @@
 <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <_LibZipSharpNugetVersion>2.0.0-alpha2</_LibZipSharpNugetVersion>
+    <_LibZipSharpNugetVersion>2.0.0-alpha4</_LibZipSharpNugetVersion>
     <_NativeBuildDir>$(MSBuildThisFileDirectory)lzsbuild</_NativeBuildDir>
     <_ExternalDir>$(MSBuildThisFileDirectory)external</_ExternalDir>
-    <_MonoPosixNugetVersion>7.0.0-alpha.21276.2</_MonoPosixNugetVersion>
+    <_MonoPosixNugetVersion>7.0.0-alpha8.21302.6</_MonoPosixNugetVersion>
     <UseXZ Condition=" '$(UseXZ)' == '' ">false</UseXZ>
   </PropertyGroup>
 </Project>

LibZipSharp/libZipSharp.csproj

@@ -6,7 +6,7 @@
     <PropertyGroup>
         <AssemblyName>libZipSharp</AssemblyName>
         <AssemblyTitle>libZipSharp</AssemblyTitle>
-        <TargetFrameworks>netstandard2.0;net45</TargetFrameworks>
+        <TargetFrameworks>netcoreapp3.1;netstandard2.0;net45</TargetFrameworks>
         <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
         <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
         <RootNamespace>Xamarin.Tools.Zip</RootNamespace>