Tests for CPP Code generation and some Python3 improvements (#961)

* Basic pytest configuration

* Added unit_test script that triggers pytest

* Changed "fixtures" to fixture_path

This is consistent with pytest's tmp_path

* Initial unit tests for esphome.helpers

* Disabled coverage reporting for esphome/components.

Focus initial unittest efforts on the core code.

* Migrated some ip_address to hypothesis

* Added a hypothesis MAC address strategy

* Initial tests for core

* Added hypothesis to requirements

* Added tests for core classes

TestTimePeriod
Lambda
ID
DocumentLocation
DocumentRange
Define
Library

* Updated test config so package root is discovered

* Setup fixtures and inital tests for pins

* Added tests for validate GPIO

* Added tests for pin type

* Added initial config_validation tests

* Added more tests for config_validation

* Added comparison unit tests

* Added repr to core.TimePeriod. Simplified identifying faults in tests

* Fixed inverted gt/lt tests

* Some tests for Espcore

* Updated syntax for Python3

* Removed usage of kwarg that isn't required

* Started writing test cases

* Started writing test cases for cpp_generator

* Additional docs and more Python3 releated improvements

* More test cases for cpp_generator.

* Fixed linter errors

* Add codegen tests to ensure file API remains stable

* Add test cases for cpp_helpers

tests/unit_tests/test_codegen.py

@@ -0,0 +1,26 @@
+import pytest
+
+from esphome import codegen as cg
+
+
+# Test interface remains the same.
+@pytest.mark.parametrize("attr", (
+    # from cpp_generator
+    "Expression", "RawExpression", "RawStatement", "TemplateArguments",
+    "StructInitializer", "ArrayInitializer", "safe_exp", "Statement", "LineComment",
+    "progmem_array", "statement", "variable", "Pvariable", "new_Pvariable",
+    "add", "add_global", "add_library", "add_build_flag", "add_define",
+    "get_variable", "get_variable_with_full_id", "process_lambda", "is_template", "templatable", "MockObj",
+    "MockObjClass",
+    # from cpp_helpers
+    "gpio_pin_expression", "register_component", "build_registry_entry",
+    "build_registry_list", "extract_registry_entry_config", "register_parented",
+    "global_ns", "void", "nullptr", "float_", "double", "bool_", "int_", "std_ns", "std_string",
+    "std_vector", "uint8", "uint16", "uint32", "int32", "const_char_ptr", "NAN",
+    "esphome_ns", "App", "Nameable", "Component", "ComponentPtr",
+    # from cpp_types
+    "PollingComponent", "Application", "optional", "arduino_json_ns", "JsonObject",
+    "JsonObjectRef", "JsonObjectConstRef", "Controller", "GPIOPin"
+))
+def test_exists(attr):
+    assert hasattr(cg, attr)