Update pip install commands to workaround new pip resolver

- lscolors dependency is now a package containing the LSCOLORS
  database and the path_colored function.
- The new pip dependency resolver finds a conflict with fill3 and
  lscolors both depending on termstr, eventhough its the same
  version of termstr??

README.md

@@ -9,13 +9,13 @@ Edit two files side by side, showing differences.
 Install diff-edit directly using pip:
 
     pip install --upgrade pip  # A recent version of pip is needed.
-    pip install git+https://github.com/ahamilton/diff-edit
+    pip install --use-deprecated=legacy-resolver git+https://github.com/ahamilton/diff-edit
 
 Or install from source:
 
     git clone https://github.com/ahamilton/diff-edit
     pip install --upgrade pip  # A recent version of pip is needed.
-    pip install ./diff-edit
+    pip install --use-deprecated=legacy-resolver ./diff-edit
 
 Then to run: