Append --pre to black installation command when pipenv is used (#5393)

* if black is being installed, append pre 
This is so it can be resolved correctly by pipenv
* extra extra read all about it!
update news
* if black is being installed make sure we pass --pre
* Thank author of PR

news/2 Fixes/5171.md

@@ -0,0 +1,2 @@
+Append `--pre` to black installation command so pipenv can properly resolve it.
+(thanks [Erin O'Connell](https://github.com/erinxocon))

src/client/common/installer/pipEnvInstaller.ts

@@ -31,8 +31,12 @@ export class PipEnvInstaller extends ModuleInstaller implements IModuleInstaller
         return interpreters && interpreters.length > 0;
     }
     protected async getExecutionInfo(moduleName: string, _resource?: Uri): Promise<ExecutionInfo> {
+        const args = ['install', moduleName, '--dev'];
+        if (moduleName === 'black') {
+            args.push('--pre');
+        };
         return {
-            args: ['install', moduleName, '--dev'],
+            args: args,
             execPath: pipenvName
         };
     }

src/test/common/installer/moduleInstaller.unit.test.ts

@@ -217,6 +217,9 @@ suite('Module Installer', () => {
                                 test(`Test args (${product.name})`, async () => {
                                     setActiveInterpreter();
                                     const expectedArgs = ['install', moduleName, '--dev'];
+                                    if (moduleName === 'black') {
+                                        expectedArgs.push('--pre')
+                                    }
                                     await installModuleAndVerifyCommand(pipenvName, expectedArgs);
                                 });
                             }