chore: force looseversion when it is used on either old or new

requirements/pypi.txt

@@ -8,16 +8,10 @@
 # should see if there is a conda package that suits your needs.
 
 pathlib2
-typing; python_version < "3.5"
-click<8.0; python_version < "3.6"
-click; python_version >= "3.6"
+click
 toml
 lexid
 colorama>=0.4
 
 # looseversion is needed to parse non PEP440 versions
-looseversion; python_version >= "3.5"
-
-# setuptools is needed for pkg_resources.parse_version
-# Support for Python 2 was dropped with v45
-setuptools<45.0.0; python_version < "3.5"
+looseversion

src/bumpver/cli.py

@@ -517,7 +517,13 @@ def _is_valid_version(raw_pattern: str, old_version: str, new_version: str) -> b
         logger.error(f"Invalid version '{new_version}' for pattern '{raw_pattern}'")
         return False
 
-    if version.parse_version(new_version) <= version.parse_version(old_version):
+    force_loose = not isinstance(
+        version.parse_version(new_version), type(version.parse_version(old_version))
+    )
+    if (
+        version.parse_version(new_version, force_loose)
+        <= version.parse_version(old_version, force_loose)
+    ):
         logger.error("Invariant violated: New version must be greater than old version ")
         logger.error(f"  Failed Invariant: '{new_version}' > '{old_version}'")
         logger.error("If the invariant is from vcs tags try '--ignore-vcs-tag' option.")

src/bumpver/version.py

@@ -9,16 +9,17 @@ import datetime as dt
 MaybeInt = typ.Optional[int]
 
 
-def parse_version(version: str) -> typ.Any:
+def parse_version(version: str, loose: bool = False) -> typ.Any:
     # pylint: disable=import-outside-toplevel; lazy import to speed up --help
 
+    if not loose:
         try:
             import pkg_resources
-
             return pkg_resources.parse_version(version)
         except (ImportError, ValueError):
-        import looseversion
+            pass
 
+    import looseversion
     return looseversion.LooseVersion(version)