Gluonts

Backporting fixes:
- Update workflow actions to latest versions 2447 by jaheba
- Simplify setup.py 2525 by jaheba
- Fix dataclass eventual handling. 2530 by jaheba
- Improve README example 2538 by lostella
- Change SeasonalNaive fallback predictor to nanmean 2549 by abdulfatir
- Compact PandasDataset string representation 2558 by lostella
- Update MANIFEST.in. 2566 by jaheba
- Add NaN validation to Evaluator 2568 by abdulfatir

Backporting fixes:
- Make serde.dataclass always kw-only. (2428 by jaheba)
- Fix `serde.dataclass` inheritance handling. (2512 by jaheba)
- Fix `QuantileForecast.quantile` in case only mean is stored (2513 by lostella)
- Remove mypy plugin for dataclass. (2514 by jaheba)
- GH Actions: Use authenticated requests for just. (2522 by jaheba)
- Fix `aggregate_valid` for non-numerical columns (2526 by lostella)

Backporting fixes:
- itertools.select. 2426 by jaheba
- Fix dataclass handling of member inheritance. 2492 by jaheba
- Fix: sort dataset keys in error message when importing non-existing dataset 2497 by lostella
- Fix DateSplitter for multiples of base frequencies 2500 by lostella
- Fix docstrings according to docformatter 2501 by lostella
- Add examples to docstring for periods_between 2504 by lostella
- Cap numpy compatibility in mxnet extra requirements 2506 by lostella
- Pin docformatter version. 2507 by jaheba
- Docs: Fix install instructions. 2508 by jaheba

What's Changed

* Backports for v0.11.5. by jaheba in https://github.com/awslabs/gluonts/pull/2491


**Full Changelog**: https://github.com/awslabs/gluonts/compare/v0.11.4...v0.11.5

Backports:

* Fix pandas issue with inferring start of X frequency. (2462 by jaheba)

Backporting fixes:
- Add test cases for PandasDataset, fix missing assertion (2453 by lostella)
- Speed up PandasDataset further (2441 by lostella)
- Fix MANIFEST.in (2456 by lostella)