refactor: parameterize the root prefix number of characters

To make i18n work on windows, we need to clean the three character
root elements(drive letter, colon, backslash) from the front of the
various paths.

Parameterize the number of chars. Old way hard coded to 1, which
leaves :/ in the path and generates bad LOCALE_DIRS.

From issues getting Roundup running on windows discussed on mailing
list by Simon Eigeldinger.

Thread starts with:

 https://sourceforge.net/p/roundup/mailman/message/41557096/

subject:  Installing Roundup on Windows 2023-10-05.

Author: rouilj

roundup/i18n.py

@@ -52,8 +52,11 @@
 # locale root is prefix/share/locale.
 if os.name == "nt":
     _mo_path = [".."] * 4 + ["share", "locale"]
+    root_prefix_chars = 3   # remove c:\ or other drive letter
 else:
     _mo_path = [".."] * 5 + ["share", "locale"]
+    root_prefix_chars = 1   # remove /
+
 _mo_path = os.path.normpath(os.path.join(msgfmt.__file__, *_mo_path))
 if _mo_path not in LOCALE_DIRS:
     LOCALE_DIRS.append(_mo_path)
@@ -67,18 +70,18 @@
 for _N in 1, 2:
     path = os.path.dirname(path)
     # path is /usr/local/lib/python3.10/site-packages
-_ldir = os.path.join(path, sys.prefix[1:], 'share', 'locale')
+_ldir = os.path.join(path, sys.prefix[root_prefix_chars:], 'share', 'locale')
 if os.path.isdir(_ldir):
     LOCALE_DIRS.append(_ldir)
 # try other places locale files are hidden on install
-_ldir = os.path.join(path, sys.prefix[1:], 'local', 'share', 'locale')
+_ldir = os.path.join(path, sys.prefix[root_prefix_chars:], 'local', 'share', 'locale')
 if os.path.isdir(_ldir):
     LOCALE_DIRS.append(_ldir)
 try:
-    _ldir = os.path.join(path, sys.base_prefix[1:], 'local', 'share', 'locale')
+    _ldir = os.path.join(path, sys.base_prefix[root_prefix_chars:], 'local', 'share', 'locale')
     if os.path.isdir(_ldir):
         LOCALE_DIRS.append(_ldir)
-    _ldir = os.path.join(path, sys.base_prefix[1:], 'share', 'locale')
+    _ldir = os.path.join(path, sys.base_prefix[root_prefix_chars:], 'share', 'locale')
     if os.path.isdir(_ldir):
         LOCALE_DIRS.append(_ldir)
 except AttributeError: