Avoid issue #21 by reverting to string variant

For now, we comment out the use of annotations from the __future__
module and use the "old" way with string notation for type annotations
refering to the 'self' class.

src/justcause/data/frames.py

@@ -1,7 +1,8 @@
 """
 Generalised DataFrame which differentiates between covariates and other column names
 """
-from __future__ import annotations
+# Uncomment only when we require Python >= 3.7
+# from __future__ import annotations
 
 from abc import ABC
 from functools import partial
@@ -69,7 +70,7 @@ def __init__(self, data, *args, **kwargs):
  self._names = dict(covariates=covariates)
 
  @property
- def _constructor(self) -> partial[CausalFrame]:
+ def _constructor(self) -> partial["CausalFrame"]:
  # This is called during operations with CausalFrames
  # We pass a marker to differentiate between explicit and implicit invocation
  kwargs = {"_internal_operation": True, **self._names}