[bindings/python] Fix and modernize the price example scripts (Bug 799796)

PR #2187 made GncPrice.get_value() return a GncNumeric wrapper and clone() a
fully wrapped GncPrice. The price example scripts still used the pre-5.15 raw
_gnc_numeric idiom and broke -- reported as Bug 799796 (cloned prices silently
kept the original value).

Example scripts (bindings/python/example_scripts/):

* quotes_historic.py -- reworked to the modern idiom; it now actually runs on
  5.x (it had further latent breakage beyond the reported one):
  - Construct each price directly with GncPrice(book) + setters instead of
    cloning an existing price, so the script no longer needs a pre-existing
    price in the book. Look the pricing currency (USD) up from the commodity
    table rather than deriving it from a split.
  - Build the value with the double->GncNumeric constructor at GnuCash's
    standard price precision -- the pricing currency's SCU * COMMODITY_DENOM_MULT
    rounded HALF_UP (gnc-pricedb.h "Price policy") -- dropping the
    Fraction.from_float().limit_denominator(100000) dance and its import.
  - Set the source with the PriceSource enum -- set_source(PriceSource.FINANCE_QUOTE)
    (the enum added in #2317). The original set_source("Finance::Quotes::Historic")
    passed a string to what has been an enum setter since 2.6, which raises
    TypeError on modern bindings.
  - Open with Session(url), which defaults to SESSION_NORMAL_OPEN. The old
    Session(url, True, False, False) passed the deprecated ignore_lock=True,
    silently bypassing the book's lock file.

* priceDB_test.py, price_database_example.py -- num/denom are methods now, so
  read v.num() / v.denom().

Prevent silent recurrence (gnucash_core.py):
The write path failed *silently*, which is how this slipped through review and
CI. num/denom are read-only accessor methods, so assigning to them merely
shadows the method and changes nothing. Add a GncNumeric.__setattr__ that raises
AttributeError on assignment to num/denom, pointing at the correct idiom
GncNumeric(num, denom).

Test (bindings/python/tests/test_numeric.py):
New test_num_denom_read_only covering the guard. It needs no backend, so it runs
in the existing python-bindings CTest on all platforms.

The wiki page Stocks/get_prices, which the reporter copied from, is being
updated separately.

bindings/python/tests/test_numeric.py

@@ -98,5 +98,18 @@ class TestGncNumeric(TestCase):
         with self.assertRaises(TypeError):
             GncNumeric(complex(1, 1))
 
+    def test_num_denom_read_only(self):
+        # num/denom are read-only accessor methods, not writable fields.
+        # Assigning to them used to silently do nothing and corrupt data
+        # (Bug 799796); it must now raise instead.
+        num = GncNumeric(1, 2)
+        with self.assertRaises(AttributeError):
+            num.num = 5
+        with self.assertRaises(AttributeError):
+            num.denom = 5
+        # The value is unchanged and the accessors still work.
+        self.assertEqual(num.num(), 1)
+        self.assertEqual(num.denom(), 2)
+
 if __name__ == '__main__':
     main()