Require that package descriptions not include newlines

[tipabu]

Previously, we would include the description-with-newlines directly as the PKG-INFO summary, which could cause subtly broken builds (for instance, the package may install, but none of the specified dependencies).

Now, raise a validation error during building, like:

RuntimeError

The Poetry configuration is invalid:
  - [description] 'First line\nSecond line (BOOOOOM)' does not match '^[^\n]+$'

Resolves: python-poetry/poetry#1372

• Added tests for changed code.
• Updated documentation for changed code.

[neersighted]

Looks pretty straightforward and solves the bug nicely. I have to admit the error message is not the best, but I'm in favor of merging this and cleaning up the error later if it's non-obvious to people, since the issue is longstanding.