The only problem I know with requirements.txt is that many people would require particular versions there while later versions work perfectly fine. Every time I clone someone's Python project to work with I have to manually replace all the =s with >=s to avoid downloading obsolete versions of the dependencies and have never encountered a problem.
Anyway, for me the most annoying thing about the Python projects architecture (and about the whole Python perhaps) is that you can't split a module into multiple files so you have to either import everything manually all over the project trying to avoid circular imports or just put everything in a single huge source file - I usually choose the latter and I hate it. The way namespaces and scopes work in C# feels just so much better.
Oh, so you weren't around when Requests went 2.0 backward-incompatible (because they changed .json() with .json, or the other way around, can't remember) and half of PyPI, with its happy-go-lucky ">=1.0", broke...?
Since then, most people have learnt that you pin first and ask questions later.
Indeed. I just hate the versions hell (as well as dealing with old versions of a language although I happen to love old hardware) so much that I've been ignoring the whole Python until the 3.6 release waiting for the time when one will be able to use all the Python stuff without bothering to learn anything about Python 2.x. It took 10 years of waiting but we are finally here now and now I enjoy Python :-)
Anyway, for me the most annoying thing about the Python projects architecture (and about the whole Python perhaps) is that you can't split a module into multiple files so you have to either import everything manually all over the project trying to avoid circular imports or just put everything in a single huge source file - I usually choose the latter and I hate it. The way namespaces and scopes work in C# feels just so much better.