Merge branch '154-nameerror-at-app-resint-live_documents-pdf-name-timedelta-is-not-defined' into 'master'

Resolve "NameError at /app/resint/live_documents/*.pdf: name 'timedelta' is not defined"

Closes #154

See merge request !212

Bump version to 2.1.dev0

See merge request !211

Resolve "Automatically create a PDF file of the substitution plan"

Closes #116

See merge request !191

Resolve "Events replace lessons"

Closes #127

See merge request !207

Update changelog from 2.0

See merge request !210

(cherry picked from commit 3dd7f9f3)

Fix changelog order

See merge request !208

Resolve "Improve the formatting of the substitution plan"

Closes #152

See merge request !206

Merge local master to new empty branch

Resolve "Substitution is shown on regular/print timetable"

Closes #149

See merge request !205

Provide fixtures for testing/demo instances

See merge request !204

Update poetry.lock

See merge request !203

Update changelog from 2.0rc3

See merge request !202

(cherry picked from commit e07e0794)

Translations update from Weblate

See merge request !186

Currently translated at 100.0% (127 of 127 strings)

Translation: AlekSIS/AlekSIS-App-Chronos
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-chronos/de/

Currently translated at 100.0% (127 of 127 strings)

Translation: AlekSIS/AlekSIS-App-Chronos
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-chronos/de/

Currently translated at 7.0% (9 of 127 strings)

Translation: AlekSIS/AlekSIS-App-Chronos
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-chronos/fr/

Update translation files

See merge request !200

Support also dates for TimePeriod's methods get_datetime_start/_end

See merge request !197

Resolve "Force __str__ methods to return strings"

Closes #151

See merge request !198

Update poetry.lock

See merge request !199