Limit Npgsql NuGet package version (see #123)

Author: peetw

NHibernate.Spatial.PostGis/NHibernate.Spatial.PostGis.csproj

@@ -29,7 +29,7 @@
 
   <ItemGroup>
     <PackageReference Include="NetTopologySuite.IO.PostGis" Version="2.0.0" />
-    <PackageReference Include="Npgsql" Version="3.2.7" />
+    <PackageReference Include="Npgsql" Version="[3.2.7, 4.0)" />
   </ItemGroup>
 
   <ItemGroup>