Deprecate astor for Python 3.9+ (#737)

Fixes #736

Author: chrisrink10

CHANGELOG.md

@@ -24,6 +24,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  * Fix issue with `ns-resolve` throwing an error on macros (#720)
  * Fix issue with py module `readerwritelock` locks handling (#722)
 
+### Removed
+ * Removed the dependency `astor` for versions of Python 3.9+ (#736)
+
 ## [v0.1.0a2]
 ### Added
  * Added support for fixtures in `basilisp.test` (#654)

pyproject.toml

@@ -30,14 +30,14 @@ include = ["README.md", "LICENSE"]
 
 [tool.poetry.dependencies]
 python = "^3.8"
-astor = "^0.8.1"
 attrs = ">=20.1.0"
 immutables = "^0.15"
 prompt-toolkit = "^3.0.0"
 pyrsistent = "^0.18.0"
 python-dateutil = "^2.8.1"
 readerwriterlock = "^1.0.8"
 
+astor = { version = "^0.8.1", python = "<3.9", optional = true }
 pytest = { version = "^6.2.5", optional = true }
 pygments = { version = "^2.9.0", optional = true }
 

src/basilisp/lang/compiler/__init__.py

@@ -1,10 +1,9 @@
 import itertools
 import os
+import sys
 import types
 from typing import Any, Callable, Iterable, List, Optional
 
-from astor import code_gen as codegen
-
 from basilisp import _pyast as ast
 from basilisp.lang import map as lmap
 from basilisp.lang import runtime as runtime
@@ -37,10 +36,33 @@
 _DEFAULT_FN = "__lisp_expr__"
 
 
-def to_py_str(t: ast.AST) -> str:
-    """Return a string of the Python code which would generate the input
-    AST node."""
-    return codegen.to_source(t)
+if sys.version_info >= (3, 9):
+    from ast import unparse
+
+    def to_py_str(t: ast.AST) -> str:
+        """Return a string of the Python code which would generate the input
+        AST node."""
+        return unparse(t) + "\n\n"
+
+else:
+    try:
+        from astor import code_gen as codegen
+
+        def to_py_str(t: ast.AST) -> str:
+            """Return a string of the Python code which would generate the input
+            AST node."""
+            return codegen.to_source(t)
+
+    except ImportError:
+        import warnings
+
+        def to_py_str(t: ast.AST) -> str:  # pylint: disable=unused-argument
+            warnings.warn(
+                "Unable to generate Python code from generated AST due to missing "
+                "dependency 'astor'",
+                RuntimeWarning,
+            )
+            return ""
 
 
 BytecodeCollector = Callable[[types.CodeType], None]