Fix loading of old versions and update changelog

[araffin]

Description

Fixes the loading of models saved with SB3 <= 0.9.0a1 (bug introduced in #138 )
and update changelog to reflect breaking changes.

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist:

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)

Note: we are using a maximum length of 127 characters per line

[Miffyli]

Ah dangit should have tested this... LGTM!
A sidenote that just appeared to me: Examples are nice, so should add get/set_parameters example like with SB2 later on.

[araffin]

Examples are nice, so should add get/set_parameters example like with SB2 later on.

Totally agree ;) Created an issues here #166