Support arbitrary list properties

[dbanty]

Closes #46.

1. Simplify the different ways of handling arrays into one ListProperty which can have any other property as its internal type. This required some less-pretty auto-naming of internally declared enums/objects (now suffixing "item") to support arbitrarily nested lists.
2. Simplify handling of property constructors. If a property has a custom constructor, it now has to be in a template. Can't be just a Python function on the class.
3. Changed constructor templates to be macros which are called with a property and source argument. property is the actual property being called, source is where the value for that property is coming from. This allows calling property constructors from within other property constructors and eliminates hard coding dict syntax.
4. Changed constructor templates to start indentation at the top level. Callers will have to handle indenting properly but it allows recursive templates where needed.
5. Simplify handling of import generation by including a get_imports function on every property. This can handle both relative and absolute imports. Removes the need to check property type before adding imports at higher levels and allows lists to include the imports of their internal properties.

[codecov]

Codecov Report

Merging #48 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #48   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          597       593    -4     
=========================================
- Hits           597       593    -4     

Impacted Files	Coverage Δ
openapi_python_client/openapi_parser/openapi.py	100.00% <100.00%> (ø)
openapi_python_client/openapi_parser/properties.py	100.00% <100.00%> (ø)

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