[MNT] changelog generator script and release workflow improvement

[fkiraly]

• add changelog generator script as used in sktime
• straighten out the release workflow so wheels build is required for test phase
• changes release to be API token based

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.14%. Comparing base (edc8329) to head (fe4e94d).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1659 +/- ## ======================================= Coverage 90.14% 90.14% ======================================= Files 32 32 Lines 4780 4780 ======================================= Hits 4309 4309 Misses 471 471 ``` | [Flag](https://app.codecov.io/gh/jdb78/pytorch-forecasting/pull/1659/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner) | Coverage Δ | | |---|---|---| | [cpu](https://app.codecov.io/gh/jdb78/pytorch-forecasting/pull/1659/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner) | `90.14% <ø> (ø)` | | | [pytest](https://app.codecov.io/gh/jdb78/pytorch-forecasting/pull/1659/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner) | `90.14% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Beitner#carryforward-flags-in-the-pull-request-comment) to find out more.

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