pip-compile --generate-hashes including bdist_egg hashes

[graingert]

Due to #1109 pip-compile now includes bdist_egg hashes when previously it did not

This isn't strictly a bug as the hashes are useless eggs that pip does not install

Environment Versions

1. OS Type N/A
2. Python version: $ python -V N/A
3. pip version: $ pip --version N/A
4. pip-tools version: $ pip-compile --version 5.1.0

Steps to replicate

 toml==0.10.0 \
     --hash=sha256:229f81c57791a41d65e399fc06bf0848bab550a9dfd5ed66df18ce5f05e73d5c \
     --hash=sha256:235682dd292d5899d361a811df37e04a8828a5b1da3115886b73cf81ebc9100e \
+    --hash=sha256:f1db651f9657708513243e61e6cc67d101a39bad662eaa9b5546f789338e07a3 \

[atugushev]

Please test/review #1122.