chore(deps): limit tortoise-orm version to >=0.21 instead of wildcard (*)

[waketzheng]

Closes #228

After try this:

make deps
pip install --upgrade "tortoise-orm<0.21"
# pip install --upgrade "tortoise-orm<0.20"
# pip install --upgrade "tortoise-orm<0.19"
# pip install --upgrade "tortoise-orm<0.18"
make test_postgres && make test_mysql && make test_sqlite && echo done.

Got ERROR:

...
======================================== 1 failed, 17 passed in 1.01s ========================================
make: *** [test_postgres] Error 1

So I limit the tortoise-orm version to '>=0.21'
and then run unittests for different torotise versions in ci.

[henadzit]

Nice!