lxml: use == not is when comparing os.name to a string literal

`lxml/lxmlGramplet.py:400` and `:758` both use:

    if os.name is 'nt':

Python 3.12+ emits a SyntaxWarning on this idiom:

    SyntaxWarning: "is" with 'str' literal. Did you mean "=="?

`is` checks object identity, not equality. The expression happens to
evaluate as expected on CPython today only because short strings get
interned during compilation, so `os.name` and the literal `'nt'` end
up pointing at the same object — but that's an implementation detail
of the interpreter and not guaranteed by the language. The right test
is value equality.

Two other call sites in the same file already use `==` for the same
comparison (`lxmlGramplet.py:161` and `:181`), so this also brings the
file's style into self-consistency.

Verified via `python3 -W error::SyntaxWarning -m py_compile
lxml/lxmlGramplet.py` (exits 0).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 2 people

lxml/lxmlGramplet.py

@@ -397,7 +397,7 @@ def read_xml(self, entry):
             os.system(f'xmllint')
 
         try:
-            if os.name is 'nt':
+            if os.name == 'nt':
                 os.system(f'xmllint --relaxng {rng} --noout {filename} {options}')
                 LOG.debug('xmllint (relaxng) : %s' % filename)
             else:
@@ -755,7 +755,7 @@ def check_valid(self, filename):
 
         dtd = os.path.join(USER_PLUGINS, 'lxml', 'grampsxml.dtd')
         try:
-            if os.name is 'nt':
+            if os.name == 'nt':
                 os.system(f'xmllint --dtdvalid {dtd} {filename} {options}')
             else:
                 os.system(f'xmllint --dtdvalid file://{dtd} {filename} {options}')