Fix installation on macOS

[beenje]

pip install "multivisor[web]==6.0.1" fails on macOS:

ERROR: Cannot install multivisor because these package versions have conflicting dependencies.

The conflict is caused by:
    supervisor-win 4.6.0 depends on pywin32==228
    supervisor-win 4.5.0 depends on pywin32==228
    supervisor-win 4.4.0 depends on pywin32
    supervisor-win 4.3.0 depends on pywin32
    supervisor-win 4.2.0 depends on pywin32
    supervisor-win 4.1.1 depends on pywin32
    supervisor-win 4.0.3 depends on pywin32
    supervisor-win 4.0.2 depends on pywin32
    supervisor-win 4.0.1 depends on pywin32
    supervisor-win 4.0.0 depends on pywin32

sys.platform returns "darwin" on macOS, so the condition "win" in sys.platform is evaluated to true and pip tries to install supervisor-win

Switch to platform.system() which makes it easier to differentiate Linux/Windows/Darwin

[tiagocoutinho]

Nice catch! Thanks for the fix.

[tiagocoutinho]

LGTM. Merging...