Fix packaging metadata and short-series validation

[Copilot]

This change removes packaging/install failures and hardens the ADF helpers against undersized inputs. It also aligns dependency metadata with valid spec syntax so the project can be installed reliably.

• Packaging metadata

  • Use README.md as the long description source.
  • Declare markdown content type explicitly.
  • Remove the stale README.rst manifest entry.
  • Normalize dependency version constraints to valid specifiers.

• Input validation

  • Reject series with 30 or fewer observations in both fracadf1 and fracadf2.
  • Fail fast with a clear ValueError instead of passing invalid truncation values downstream.

Example:

fracadf1(X)  # raises ValueError if len(X) <= 30
fracadf2(X)  # raises ValueError if len(X) <= 30

[ulf1]

set to statsmodels>=0.11,<1

[ulf1]

set to statsmodels>=0.11,<1