Update default setup to use parallelized dep installation

Move deprecation warnings to the precondition function
Deprecate --parallelize-locked-install option
Add --parallel-install-threads option

tox_poetry_installer/constants.py

@@ -21,3 +21,6 @@ REPORTER_PREFIX: str = f"{__about__.__title__}:"
 
 # Internal list of packages that poetry has deemed unsafe and are excluded from the lockfile
 UNSAFE_PACKAGES: Set[str] = {"distribute", "pip", "setuptools", "wheel"}
+
+# Number of threads to use for installing dependencies by default
+DEFAULT_INSTALL_THREADS: int = 10