Remove more python 3.7 workarounds

Author: chadrik

src/rez/plugin_managers.py

@@ -14,7 +14,7 @@
 from rez.utils.logging_ import print_debug, print_warning
 from rez.exceptions import RezPluginError
 from zipimport import zipimporter
-from typing import overload, Any, TypeVar, TYPE_CHECKING
+from typing import overload, Any, Literal, TypeVar, TYPE_CHECKING
 import pkgutil
 import os.path
 import sys
@@ -26,7 +26,6 @@
     from rez.vendor.importlib_metadata import entry_points
 
 if TYPE_CHECKING:
-    from typing import Literal  # not available in typing module until 3.8
     from rez.shells import Shell
     from rez.release_vcs import ReleaseVCS
     from rez.release_hook import ReleaseHook

src/rezplugins/shell/sh.py

@@ -17,10 +17,7 @@
 from rez.rex import EscapedString
 from shlex import quote
 
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-    from typing import Literal
+from typing import Literal
 
 
 class SH(UnixShell):

src/rezplugins/shell/zsh.py

@@ -16,10 +16,7 @@
 from rez import module_root_path
 from shlex import quote
 
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-    from typing import Literal
+from typing import Literal
 
 
 class Zsh(SH):