Python API implementation (#2195)

* Draft implementation

* Fix style

* Add api tests

* Fix lint

* Update docs

* Update tests

* Set env

* Fixup

* Fixup

* Fix lint

* Revert

tests/inference_tests/test_python_api.py

@@ -0,0 +1,36 @@
+import os
+import unittest
+
+from tests import get_tests_output_path
+from TTS.api import TTS
+
+OUTPUT_PATH = os.path.join(get_tests_output_path(), "test_python_api.wav")
+
+
+class TTSTest(unittest.TestCase):
+    def test_single_speaker_model(self):
+        tts = TTS(model_name="tts_models/de/thorsten/tacotron2-DDC", progress_bar=False, gpu=False)
+
+        error_raised = False
+        try:
+            tts.tts_to_file(text="Ich bin eine Testnachricht.", speaker="Thorsten", language="de")
+        except ValueError:
+            error_raised = True
+
+        tts.tts_to_file(text="Ich bin eine Testnachricht.", file_path=OUTPUT_PATH)
+
+        self.assertTrue(error_raised)
+        self.assertFalse(tts.is_multi_speaker)
+        self.assertFalse(tts.is_multi_lingual)
+        self.assertIsNone(tts.speakers)
+        self.assertIsNone(tts.languages)
+
+    def test_multi_speaker_multi_lingual_model(self):
+        tts = TTS()
+        tts.load_model_by_name(tts.models[0])  # YourTTS
+        tts.tts_to_file(text="Hello world!", speaker=tts.speakers[0], language=tts.languages[0], file_path=OUTPUT_PATH)
+
+        self.assertTrue(tts.is_multi_speaker)
+        self.assertTrue(tts.is_multi_lingual)
+        self.assertGreater(len(tts.speakers), 1)
+        self.assertGreater(len(tts.languages), 1)