ENH: Add PdfReader.pdf_header (#1013)

The new attribute will return the first bytes of the PDF file. This is typically something like `"%PDF-1.4"`.
That can be used to get the PDF version of the file - at least the version the file claims to have.

PyPDF2/_reader.py

@@ -257,6 +257,14 @@ def __init__(
         if password is not None and self.decrypt(password) == 0:
             raise PdfReadError("Wrong password")
 
+    @property
+    def pdf_header(self) -> str:
+        loc = self.stream.tell()
+        self.stream.seek(0, 0)
+        pdf_file_version = self.stream.read(8).decode("utf-8")
+        self.stream.seek(loc, 0)  # return to where it was
+        return pdf_file_version
+
     @property
     def metadata(self) -> Optional[DocumentInformation]:
         """

tests/test_reader.py

@@ -821,3 +821,16 @@ def test_unexpected_destination():
     with pytest.raises(PdfReadError) as exc:
         merger.append(reader)
     assert exc.value.args[0] == "Unexpected destination '/1'"
+
+
+@pytest.mark.parametrize(
+    "src,pdf_header",
+    [
+        (os.path.join(RESOURCE_ROOT, "attachment.pdf"), "%PDF-1.5"),
+        (os.path.join(RESOURCE_ROOT, "crazyones.pdf"), "%PDF-1.5"),
+    ],
+)
+def test_header(src, pdf_header):
+    reader = PdfReader(src)
+
+    assert reader.pdf_header == pdf_header