Fix: Update regex matching to handle proper class escaping

On Python 3.12 it's throwing compile warnings on the regex compiles:

```
spdx/verify-spdx-headers:8: SyntaxWarning: invalid escape sequence '\s'
  SPDX = re.compile(f'SPDX-License-Identifier:\s+({SLUG.pattern})')
spdx/verify-spdx-headers:21: SyntaxWarning: invalid escape sequence '\s'
  pattern = f"^{init}\s*{SPDX.pattern}\s*{fini}\\s*$"
spdx/verify-spdx-headers:21: SyntaxWarning: invalid escape sequence '\s'
  pattern = f"^{init}\s*{SPDX.pattern}\s*{fini}\s*$"
```

This escapes the the '\s' to be '\\s' instead as that is compliant,
seems to compile correctly, and makes the warning go away.  Quick
checking on my own repos seemed to not break anything.

Signed-off-by: John 'Warthog9' Hawley <[email protected]>

Author: warthog9

verify-spdx-headers

@@ -5,7 +5,7 @@ import os
 import re
 
 SLUG = re.compile('[a-zA-Z0-9.-]+')
-SPDX = re.compile(f'SPDX-License-Identifier:\s+({SLUG.pattern})')
+SPDX = re.compile(f'SPDX-License-Identifier:\\s+({SLUG.pattern})')
 
 class Language:
     def __init__(self, *comments, shebang=False):
@@ -18,7 +18,7 @@ class Language:
             if isinstance(comment, tuple):
                 (init, fini) = comment
 
-            pattern = f"^{init}\s*{SPDX.pattern}\s*{fini}\s*$"
+            pattern = f"^{init}\\s*{SPDX.pattern}\\s*{fini}\\s*$"
             self.__match.append(re.compile(pattern))
 
     def license(self, path):