Fix a simultaneous file access error in Director when generating sources

[dendron2000]

When trying to create a source package using thespian.Director on Windows I got this error:

PermissionError: [WinError 32] The process cannot access the file because it is being used by another process

It seems there is a blunder in the source code which tries to rename a file while keeping it open inside the context (by using the "with" statement).

[kquick]

Oh good catch, thank you!