python3Packages.mypy: don't use mypyc on 32-bit platforms

Almost all i686-linux tests got blocked because of this problem:
  https://hydra.nixos.org/eval/1638038#tabs-now-fail
It regressed in PR #105462 (commit ad26cb9ee8).

Now I tested that at least some test got fixed:
  nix build -f nixos/release-combined.nix nixos.tests.knot.i686-linux
This change won't even cause any rebuild on 64-bit platforms,
and using nix booleans seems nicer anyway.

pkgs/development/python-modules/mypy/default.nix

@@ -53,7 +53,8 @@ buildPythonPackage rec {
 
   # Compile mypy with mypyc, which makes mypy about 4 times faster. The compiled
   # version is also the default in the wheels on Pypi that include binaries.
-  MYPY_USE_MYPYC = "1";
+  # is64bit: unfortunately the build would exhaust all possible memory on i686-linux.
+  MYPY_USE_MYPYC = stdenv.buildPlatform.is64bit;
 
   meta = with stdenv.lib; {
     description = "Optional static typing for Python";