pin flate to v1.0.26, h2 to v0.3.20, reqwest to v0.11.18, petgraph to v0.6.3

flate 1.0.27 - rust-lang/rust#86985
reqwest 0.11.19 requires h2 v0.3.21 which requires rust 1.63.0
petgraph 0.6.4 requires indexmap 2.0.0 which requires hashbrown 0.14.0 and hence rust 1.64.0

Author: nyetwurk