Flush NamedTemporaryFile before reading

We have to flush the file to make sure that wirte is complete
before we re-open it and read from it. Otherwise, we are
not guaranteed to read the data back. This can lead to test
failures on certain systems.

Author: iv-m

sympy/parsing/c/c_parser.py

@@ -180,6 +180,7 @@ def parse_str(self, source, flags):
             """
             file = tempfile.NamedTemporaryFile(mode = 'w+', suffix = '.cpp')
             file.write(source)
+            file.flush()
             file.seek(0)
             self.tu = self.index.parse(
                 file.name,

sympy/parsing/tests/test_custom_latex.py

@@ -30,6 +30,7 @@ def init_custom_parser(modification, transformer=None):
 
     with tempfile.NamedTemporaryFile() as f:
         f.write(bytes(latex_grammar, encoding="utf8"))
+        f.flush()
 
         parser = LarkLaTeXParser(grammar_file=f.name, transformer=transformer)