Add proxy support for pypi package managers (#150)

* initial pypi registry implementation

* support proxy mode for pypi package managers

* support proxy mode for pypi package managers - 2

* rm default mode as proxy for pip3

* update goproxy version & fix pypi proxy failing on 304

* add PIP_RETRIES=0 env

* update pmg e2e & add proxy mode e2e for pypi

* rm safedep-test-pkg for pypi proxy e2e

proxy/interceptors/factory.go

@@ -43,6 +43,14 @@ func (f *InterceptorFactory) CreateInterceptor(ecosystem packagev1.Ecosystem) (p
 			f.confirmationChan,
 		), nil
 
+	case packagev1.Ecosystem_ECOSYSTEM_PYPI:
+		return NewPypiRegistryInterceptor(
+			f.analyzer,
+			f.cache,
+			f.statsCollector,
+			f.confirmationChan,
+		), nil
+
 	default:
 		return nil, fmt.Errorf("proxy-based interception not yet supported for ecosystem: %s", ecosystem.String())
 	}
@@ -52,6 +60,7 @@ func (f *InterceptorFactory) CreateInterceptor(ecosystem packagev1.Ecosystem) (p
 func SupportedEcosystems() []packagev1.Ecosystem {
 	return []packagev1.Ecosystem{
 		packagev1.Ecosystem_ECOSYSTEM_NPM,
+		packagev1.Ecosystem_ECOSYSTEM_PYPI,
 	}
 }