Also classify .in as valid requirements file

[mering]

The .in extension is often used when using pip-compile to "compile" the .in to .txt. It should be treated as valid in

pip-upgrader/pip_upgrader/requirements_detector.py

Line 47 in 87da550

extension_ok = filename.endswith('txt') or filename.endswith('pip')

Currently there is no way to tell pip-upgrader to work on such files.

Furthermore, I don't understand why you are checking file extensions at all in the case one is explicitly specifying a file via command line argument.