environment: Add a few type annotations

These are just annotations in code that I'm working for this series

mesonbuild/environment.py

@@ -359,7 +359,7 @@ def detect_cpu_family(compilers: CompilersDict) -> str:
 
     return trial
 
-def detect_cpu(compilers: CompilersDict):
+def detect_cpu(compilers: CompilersDict) -> str:
     if mesonlib.is_windows():
         trial = detect_windows_arch(compilers)
     elif mesonlib.is_freebsd() or mesonlib.is_netbsd() or mesonlib.is_openbsd() or mesonlib.is_aix():
@@ -392,15 +392,13 @@ def detect_cpu(compilers: CompilersDict):
     # detect_cpu_family() above.
     return trial
 
-def detect_system():
+def detect_system() -> str:
     if sys.platform == 'cygwin':
         return 'cygwin'
     return platform.system().lower()
 
-def detect_msys2_arch():
-    if 'MSYSTEM_CARCH' in os.environ:
-        return os.environ['MSYSTEM_CARCH']
-    return None
+def detect_msys2_arch() -> T.Optional[str]:
+    return os.environ.get('MSYSTEM_CARCH', None)
 
 def detect_machine_info(compilers: T.Optional[CompilersDict] = None) -> MachineInfo:
     """Detect the machine we're running on