Updates to the library to make the Money, Rate and Number objects copyable with copy.copy / copy.deepcopy

[kalaspuff]

>>> from stockholm import Money, Number
>>> import copy
>>> 
>>> lst = [Money(4711, "SEK"), Money(0.01, "SEK"), Number(42)]
>>> 
>>> copy.copy(lst)
[<stockholm.Money: "4711.00 SEK">, <stockholm.Money: "0.01 SEK">, <stockholm.Number: "42">]
>>> copy.deepcopy(lst)
[<stockholm.Money: "4711.00 SEK">, <stockholm.Money: "0.01 SEK">, <stockholm.Number: "42">]

>>> from stockholm import Money, Number
>>> import copy
>>> 
>>> d = {"values": [Number(1338.9999999), Money("499.99 USD")]}
>>> 
>>> copy.copy(d)
{'values': [<stockholm.Number: "1338.999999999">, <stockholm.Money: "499.99 USD">]}
>>> copy.deepcopy(d)
{'values': [<stockholm.Number: "1338.999999999">, <stockholm.Money: "499.99 USD">]}

[codecov]

Codecov Report

Merging #243 (47f3c23) into master (283ebc7) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #243   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines         1818      1810    -8     
=========================================
- Hits          1818      1810    -8     

Files Changed	Coverage Δ
stockholm/money.py	100.00% <100.00%> (ø)
stockholm/rate.py	100.00% <100.00%> (ø)