Implement platform_system in requirements.txt

Pip uses the following notation for platform-specific modules:
```
pywin32;platform_system=='Windows'
pyudev;platform_system=='Linux'
```
`circup install -r` would pick those as `pywin32;platform_system`, etc.

Would it be a good idea to:
- have circup find and read the platform_system parameter (and ignore the module)
- use a `platform_system=='circuitpython'` to specify modules that pip will ignore, but circup still install

This would allow pip and circup requirements to live in the same file to some extent for, say a project that is to be deployed on both a host and a target board. (For example circup itself can be a requirement for the host). Modules without a platform_system specified would still be installed (or ignored if known) as usual.

Start by tracing how `circup install -r` reads `requirements.txt` and handles entries containing platform markers. Compare the requested `platform_system` behavior for Windows, Linux, and CircuitPython with the current parsing flow; done means host-only entries are ignored appropriately while CircuitPython entries remain installable.

Written by the indexing model from the issue text.