Don't build aarch64 wheels for now

The porting of arm64 builds to Github Actions uncovered an
architecture-specific (or, perhaps, virtualization-specific) bug in
asyncpg, so, rather than blocking the release, drop aarch64 wheels for
now.  Also, `manylinux2014_aarch64` is not considered stable yet
(see pypa/manylinux#84) and so the compiled wheels might not even work
correctly on all aarch64 boxes.

.github/workflows/release.yml

@@ -74,7 +74,7 @@ jobs:
       matrix:
         python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
         os: [ubuntu-20.04, macos-latest, windows-latest]
-        arch: [x86_64, aarch64]
+        arch: [x86_64]
         exclude:
           # Python 3.5 is unable to properly
           # find the recent VS tooling