Colored graph of 7-day incidence rate

In this release:

• add colored graph of 7-day incidence rate: #213, #224, #232, #233
• add python-3.9 support: #226
• introduce an uncertainty region for graphs of cases and deaths: #230, #233
• backup of the RKI population data: #262, #265, #266
• MPSD in acknowledgements #244
• fix: the population of US regions was calculated incorrectly #238
• fix: FutureWarning related to the updates in pandas-1.3.0 #258
• fix: nbconvert issue: #246, #249
• other fixes: #220, #221, #227, #235, #237, #260