Add exe suffix for pip on Windows

[chrzyki]

requirements.txt was empty on Windows platform due to the fact that pip could not be found. On Windows, the 'script' is not pip but rather pip.exe. See also lexibank/pylexibank#272 (comment).

[chrzyki]

Just noted that on Windows platforms you seem to get an additional \n in the requirements, i.e.:

appdirs==1.4.4

attrs==23.1.0

Babel==2.13.1

bs4==0.0.1

...

Not yet sure where that comes from but I can have a look. However, I'm not sure whether this issue is problematic enough to potentially introduce more platform-specific code.