Add GF2 type for Galois field elements

- add `GF2` class in the `lalib.elements` sub-package
  implementing a typical Galois field with two elements
- the singleton objects `one` and `zero` are the concrete
  instances of the `GF2` type for the end users
- besides the typical Galois arithmetic, `one` and `zero`
  behave like the built-in numbers `1` and `0`
  and implement the `numbers.Rational` interface
- add exhaustive docstrings with usage examples
- add (unit) test cases with 100% coverage

pyproject.toml

@@ -30,6 +30,8 @@ repository = "https://github.com/webartifex/lalib"
 
 python = "^3.9"
 
+typing-extensions = [ { python = "<3.11", version = "^4.12" } ]  # to support Python 3.9 & 3.10
+
 
 [tool.poetry.group.dev.dependencies]
 
@@ -127,6 +129,9 @@ exclude_lines = [
     # "pragma: no cover"
     # => Intentionally commented out as we thrive for 100% test coverage
 
+    # PyPI's "typing-extensions" are needed to make `mypy` work
+    "pragma: no cover ${PRAGMA_SUPPORT_39_N_310}",
+
 ]