Update pytz verion

Currently labelstudio depends on pytz==2019.3
This causes issues with other packages for eg. latest version of pandas which requires pytz>=2020.1 or celery which requires pytz>=2021.3

Please update pytz dependency.

**To Reproduce**
```
poetry init
poetry add celery pandas label-studio
```

```
❯ poetry add celery pandas label-studio
Using version ^5.2.7 for celery
Using version ^1.4.2 for pandas
Using version ^1.4.1 for label-studio

Updating dependencies
Resolving dependencies... (0.7s)

SolverProblemError

Because no versions of label-studio match >1.4.1,<1.4.1.post0 || >1.4.1.post0,<1.4.1.post1 || >1.4.1.post1,<2.0.0
and label-studio (1.4.1) depends on pytz (2019.3), label-studio (>=1.4.1,<1.4.1.post0 || >1.4.1.post0,<1.4.1.post1 || >1.4.1.post1,<2.0.0) requires pytz (2019.3).
And because label-studio (1.4.1.post0) depends on pytz (2019.3)
and label-studio (1.4.1.post1) depends on pytz (2019.3), label-studio (>=1.4.1,<2.0.0) requires pytz (2019.3).
And because pandas (1.4.2) depends on pytz (>=2020.1)
and no versions of pandas match >1.4.2,<2.0.0, label-studio (>=1.4.1,<2.0.0) is incompatible with pandas (>=1.4.2,<2.0.0).
So, because test-labelstudio depends on both pandas (^1.4.2) and label-studio (^1.4.1), version solving failed.
```

**Environment (please complete the following information):**
- OS: Linux