chore: import upstream snapshot with attribution

basics/base_pe.py

@@ -0,0 +1,7 @@
+class BasePE:
+    def get_pitch(
+            self, waveform, samplerate, length,
+            *, hop_size, f0_min=65, f0_max=1100,
+            speed=1, interp_uv=False
+    ):
+        raise NotImplementedError()