  The orbital eccentricity of a merging binary black hole leaves an imprint on
the associated gravitational-wave signal that can reveal whether the binary
formed in isolation or in a dynamical environment, such as the core of a dense
star cluster. We present measurements of the eccentricity of 26 binary black
hole mergers in the second LIGO--Virgo gravitational-wave transient catalog,
updating the total number of binary black holes analysed for orbital
eccentricity to 36. Using the \texttt{SEOBNRE} waveform, we find the data for
GW190620A is poorly explained by the zero-eccentricity hypothesis (frequentist
$p$-value $\lesssim 0.1\%$). Using a log-uniform prior on eccentricity, the
eccentricity at $\unit[10]{Hz}$ for GW190620A is constrained to
$e_{10}\geq0.05$ ($0.1$) at $74\%$ ($65\%$) credibility. With this log-uniform
prior, we obtain a $90\%$ credible lower eccentricity limit of $0.001$, while
assuming a uniform prior leads the data to prefer $e_{10} \geq 0.11$ at $90\%$
credibility. This is the second measurement of a binary black hole system with
statistical support for non-zero eccentricity; the intermediate-mass black hole
merger GW190521 was the first. Interpretation of these two events is currently
complicated by waveform systematics; we are unable to simultaneously model the
effects of relativistic precession and eccentricity. However, if these two
events are, in fact, eccentric mergers, then there are potentially many more
dynamically assembled mergers in the LIGO--Virgo catalog without measurable
eccentricity; $\gtrsim 27\%$ of the observed LIGO--Virgo binaries may have been
assembled dynamically in dense stellar environments ($95\%$ credibility).
