TST: Skip tests using fpdf2 if it's not installed

The Debian ecosystem installs only packages which are also present
in the Debian ecosystem for testing. This includes pytest, but it
does not include fpdf2.

Adding the line 'pytest.importorskip("fpdf")' within the test
and before the import ensures that pytest skips the test in
case fpdf is not installed.

Closes #2408

tests/test_page.py

@@ -1270,6 +1270,7 @@ def test_compression():
     """Test for issue #1897"""
 
     def create_stamp_pdf() -> BytesIO:
+        pytest.importorskip("fpdf")
         from fpdf import FPDF
 
         pdf = FPDF()

tests/test_writer.py

@@ -1951,6 +1951,7 @@ def _get_parent_bookmark(current_indent, history_indent, bookmarks) -> Any:
 
 def test_merging_many_temporary_files():
     def create_number_pdf(n) -> BytesIO:
+        pytest.importorskip("fpdf")
         from fpdf import FPDF
 
         pdf = FPDF()