Upgrade projects to .NET Core 3.1 (microsoft#1961)

* Remove stale reference

* Don't suppress LHS diagnostics on augmented assign

* Revert "Don't suppress LHS diagnostics on augmented assign"

This reverts commit 6109ac7.

* Escape [ and ]

* PR feedback

* Upgrade projects to .NET Core 3.1

Author: Mikhail Arkhipov

src/Analysis/Ast/Test/Microsoft.Python.Analysis.Tests.csproj

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <RootNamespace>Microsoft.Python.Analysis.Tests</RootNamespace>
     <AssemblyName>Microsoft.Python.Analysis.Tests</AssemblyName>
   </PropertyGroup>

src/Caching/Test/Microsoft.Python.Analysis.Caching.Tests.csproj

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <RootNamespace>Microsoft.Python.Analysis.Caching.Tests</RootNamespace>
     <AssemblyName>Microsoft.Python.Analysis.Caching.Tests</AssemblyName>
   </PropertyGroup>

src/Core/Test/Microsoft.Python.Core.Tests.csproj

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <RootNamespace>Microsoft.Python.Core.Tests</RootNamespace>
     <AssemblyName>Microsoft.Python.Core.Tests</AssemblyName>
   </PropertyGroup>

src/LanguageServer/Impl/Microsoft.Python.LanguageServer.csproj

@@ -1,6 +1,6 @@
 <Project>
     <PropertyGroup>
-        <TargetFramework>netcoreapp3.0</TargetFramework>
+        <TargetFramework>netcoreapp3.1</TargetFramework>
         <RootNamespace>Microsoft.Python.LanguageServer</RootNamespace>
         <AssemblyName>Microsoft.Python.LanguageServer</AssemblyName>
     </PropertyGroup>

src/LanguageServer/Test/Microsoft.Python.LanguageServer.Tests.csproj

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <RootNamespace>Microsoft.Python.LanguageServer.Tests</RootNamespace>
     <AssemblyName>Microsoft.Python.LanguageServer.Tests</AssemblyName>
   </PropertyGroup>