Import platform later

It takes a while to import
platform, so use sys.platform
to check for macOS/Linux, and
only import platform to detect
BSD dialects.

Author: pjonsson

typer/main.py

@@ -1,6 +1,5 @@
 import inspect
 import os
-import platform
 import shutil
 import subprocess
 import sys
@@ -1072,13 +1071,15 @@ def run(function: Callable[..., Any]) -> None:
 
 
 def _is_macos() -> bool:
-    return platform.system() == "Darwin"
+    return sys.platform == "darwin"
 
 
 def _is_linux_or_bsd() -> bool:
-    if platform.system() == "Linux":
+    if sys.platform == "linux":
         return True
 
+    import platform
+
     return "BSD" in platform.system()