The ClearCase SCM backend invokes `cleartool` a lot, and every time was assembling a big Popen call and duplicating error handling. This change centralizes all of that into a single method, while also improving unicode/bytes compatibility, logging, and console shell handling for windows. The windows issue is the most interesting one here. Some windows applications are marked as requiring a console (as opposed to using windows), and invoking them would always pop up a console window, regardless of whether input and output was actually possible. `cleartool` is one such executable. An existing workaround had been added to use `shell=True`, but this was only enabled when using Python 2.7 and Windows 7 (the initial patch was not written with future-proofing in mind), and relying on the shell opens up a variety of other potential issues, including security concerns. The `startupinfo` flag to `Popen` allows actually controlling this at a lower level. Testing Done: - Ran unit tests. - startupinfo changes haven't yet been tested because I don't have a windows environment set up at the moment, but I'll make sure to verify its correctness before landing this.