Fix shiny run for Windows absolute paths with drive letter

[wch]

Previously, using shiny run with a Windows path with a drive letter would result in an error. For example:

C:\Users\Jon\Documents\PyShinyTest>shiny run C:/Users/Jon/Documents/PyShinyTest/app.py
INFO Will watch for changes in these directories: ['C:\Users\Jon\Documents\PyShinyTest']
INFO Uvicorn running on http://127.0.0.1:9092 (Press CTRL+C to quit)
INFO Started reloader process 19484 using StatReload
ERROR: Error loading ASGI app. Could not import module "C".
INFO Stopping reloader process 19484

This PR fixes the issue.