Make Pydantic a required dependency (astronomer#939)

The pull request astronomer#736 initially designated Pydantic as an 
optional dependency. However, a subsequent pull request, 
astronomer#794 as an [import](https://github.com/astronomer/astronomer-cosmos/pull/794/files#diff-bd3fa47d7a9b96d7bb365f3ba3b60eaf0b20e06a48b28814cf6f6e6fb64d4da6R12),
introduced an implementation that requires Pydantic
to be a mandatory dependency for astronomer-cosmos. 

This pull request addresses the bug encountered while 
running `airflow db init`, as described in astronomer#936, by enforcing 
Pydantic as a required dependency.

Closes: astronomer#936

pyproject.toml

@@ -32,6 +32,7 @@ dependencies = [
     "apache-airflow>=2.3.0",
     "importlib-metadata; python_version < '3.8'",
     "Jinja2>=3.0.0",
+    "pydantic>=1.10.0",
     "typing-extensions; python_version < '3.8'",
     "virtualenv",
 ]
@@ -85,9 +86,6 @@ docker = [
 kubernetes = [
     "apache-airflow-providers-cncf-kubernetes>=5.1.1",
 ]
-pydantic = [
-        "pydantic>=1.10.0",
-]
 azure-container-instance = [
     "apache-airflow-providers-microsoft-azure>=8.4.0",
 ]