Consider removing `uvloop` from the `accel` extra deps

Currently, uvloop (=^0.16.0 for 0.18.0, =^0.14.0 for 0.16.19+) is in the accel extra dependencies. But tortoise-orm cannot "install" it by itself, uvloop must be "installed" by users' codes. Means that uvloop is always an immediate dependence for their codes and should be listed in requirements.txt (or so on). In such a situation, uvloop in the accel extra dependences is so embarrassed.

What's more, uvloop is a drop-in replacement of the built-in asyncio event loop. There should not be any breaking changes for its upcoming versions unless the asyncio event loop has ones. Thus, its version does not matter as long as it is compatible with users' Python version and of no security vulnerabilities found. Tortoise-orm does not bump uvloop frequently, which means that if someone needs a newer version of uvloop (the left-most non-zero digit bumped), they cannot mix it with tortoise-orm[accel], but only to manually mix the newer version of uvloop and other accel dependencies farcically.

Start by locating the accel extra dependency declaration and compare it with the requirements.txt dependency arrangement mentioned in the issue. Done means uvloop is no longer included in the accel extra while the remaining accel dependencies continue to work.