Reduce import overhead of uuid module

This follows in the footsteps of python#21586
This speeds up import uuid by about 6ms on Linux.

Before:
```
λ hyperfine -w 4 "./python -c 'import uuid'"
Benchmark 1: ./python -c 'import uuid'
  Time (mean ± σ):      20.4 ms ±   0.4 ms    [User: 16.7 ms, System: 3.8 ms]
  Range (min … max):    19.6 ms …  21.8 ms    136 runs
```
After:
```
λ hyperfine -w 4 "./python -c 'import uuid'"
Benchmark 1: ./python -c 'import uuid'
  Time (mean ± σ):      14.5 ms ±   0.3 ms    [User: 11.5 ms, System: 3.2 ms]
  Range (min … max):    13.9 ms …  16.0 ms    175 runs
```

Lib/uuid.py

@@ -53,8 +53,11 @@
 __author__ = 'Ka-Ping Yee <ping@zesty.ca>'
 
 # The recognized platforms - known behaviors
-if sys.platform in ('win32', 'darwin', 'emscripten', 'wasi'):
+if sys.platform in {'win32', 'darwin', 'emscripten', 'wasi'}:
     _AIX = _LINUX = False
+elif sys.platform == 'linux':
+    _LINUX = True
+    _AIX = False
 else:
     import platform
     _platform_system = platform.system()