Add a hint with the correct version specifier (#25)

when version is rejected due to being npm-style, add a hint

Author: DanielSchiavini

tests/test_versioning.py

@@ -56,7 +56,8 @@ def test_version_does_not_exist():
 
 def test_npm_version_for_04_release():
     with pytest.raises(UnexpectedVersionError) as excinfo:
-        detect_vyper_version_from_source("# pragma version ^0.4.0")
+        detect_vyper_version_from_source("# pragma version ^0.4.1")
 
-    expected_msg = "Please use the pypi-style version specifier for vyper versions >= 0.4.0"
+    expected_msg = "Please use the pypi-style version specifier "
+    expected_msg += "for vyper versions >= 0.4.0 (hint: try ~=0.4.1)"
     assert str(excinfo.value) == expected_msg

vvm/utils/versioning.py

@@ -32,7 +32,8 @@ def _detect_version_specifier(source_code: str) -> Specifier:
     specifier, version_str = match.groups()
     if specifier in ("~", "^"):  # convert from npm-style to pypi-style
         if Version(version_str) >= Version("0.4.0"):
-            error = "Please use the pypi-style version specifier for vyper versions >= 0.4.0"
+            error = "Please use the pypi-style version specifier "
+            error += f"for vyper versions >= 0.4.0 (hint: try ~={version_str})"
             raise UnexpectedVersionError(error)
         # for v0.x, both specifiers are equivalent
         specifier = "~="  # finds compatible versions