Use typing-extensions on py<3.8

typing-extensions backport typing features from Python 3.8.  Use
built-in typing module in Python 3.8 and newer.

examples/top_lite_simulator.py

@@ -9,7 +9,11 @@
 from rich.console import Console
 from rich.live import Live
 from rich.table import Table
-from typing_extensions import Literal
+
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal
 
 
 @dataclass

pyproject.toml

@@ -25,7 +25,7 @@ include = ["rich/py.typed"]
 
 [tool.poetry.dependencies]
 python = "^3.6"
-typing-extensions = "^3.7.4"
+typing-extensions = {version = "^3.7.4", python = "<3.8"}
 dataclasses = {version=">=0.7,<0.9", python = "~3.6"} 
 pygments = "^2.6.0"
 commonmark = "^0.9.0"

rich/_ratio.py

@@ -1,7 +1,11 @@
 from fractions import Fraction
 from math import ceil, floor, modf
 from typing import cast, List, Optional, Sequence
-from typing_extensions import Protocol
+
+try:
+    from typing import Protocol
+except ImportError:
+    from typing_extensions import Protocol
 
 
 class Edge(Protocol):

rich/align.py

@@ -1,7 +1,11 @@
 from itertools import chain
 from typing import Iterable, Optional, TYPE_CHECKING
 
-from typing_extensions import Literal
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal
+
 from .constrain import Constrain
 from .jupyter import JupyterMixin
 from .measure import Measurement

rich/box.py

@@ -1,6 +1,9 @@
 from typing import TYPE_CHECKING, Iterable, List
 
-from typing_extensions import Literal
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal
 
 from ._loop import loop_last
 

rich/console.py

@@ -28,7 +28,10 @@
     cast,
 )
 
-from typing_extensions import Literal, Protocol, runtime_checkable
+try:
+    from typing import Literal, Protocol, runtime_checkable
+except ImportError:
+    from typing_extensions import Literal, Protocol, runtime_checkable
 
 from . import errors, themes
 from ._emoji_replace import _emoji_replace

rich/live_render.py

@@ -1,7 +1,10 @@
 from threading import RLock
 from typing import Optional, Tuple
 
-from typing_extensions import Literal
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal
 
 from ._loop import loop_last
 from .console import Console, ConsoleOptions, RenderableType, RenderResult