Make the segments backend optional so it can be omitted when only
the espeak backend is needed, avoiding the csvw/rdflib dependency chain.

--- a/phonemizer/backend/__init__.py
+++ b/phonemizer/backend/__init__.py
@@ -19,9 +19,15 @@
 from .espeak.espeak import EspeakBackend
 from .espeak.mbrola import EspeakMbrolaBackend
 from .festival.festival import FestivalBackend
-from .segments import SegmentsBackend

+try:
+    from .segments import SegmentsBackend
+except ImportError:
+    SegmentsBackend = None

-BACKENDS = {b.name(): b for b in (
-    EspeakBackend, FestivalBackend, SegmentsBackend, EspeakMbrolaBackend)}
+
+_backends = [EspeakBackend, FestivalBackend, EspeakMbrolaBackend]
+if SegmentsBackend is not None:
+    _backends.append(SegmentsBackend)
+BACKENDS = {b.name(): b for b in _backends}
 """The different phonemization backends as a mapping (name, class)"""