Bump pytz from 2018.3 to 2020.1 in /cvat/requirements (#1717)

Bumps [pytz](https://github.com/stub42/pytz) from 2018.3 to 2020.1.
- [Release notes](https://github.com/stub42/pytz/releases)
- [Commits](https://github.com/stub42/pytz/compare/release_2018.3...release_2020.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

cvat/requirements/base.txt

@@ -9,7 +9,7 @@ EasyProcess==0.2.3
 Pillow==7.1.2
 numpy==1.18.5
 python-ldap==3.2.0
-pytz==2018.3
+pytz==2020.1
 pyunpack==0.1.2
 rcssmin==1.0.6
 redis==3.2.0