Declare activities.csv's date format to prevent Pandas getting confused if the first was in May

[hugovk]

Fixes https://github.com/marcusvolz/strava_py/issues/30 and includes the example activities.csv from there as a test file.

We also need to use Pandas < 2 because calmap does not yet support version 2. There's a fix pending at:

• https://github.com/MarvinT/calmap/pull/14